My biggest customer has stopped paying me. They currently owe about $70,000, debts accumulated over the past 18 months. 
 
The customer has given no reason for stopping payments. They do not appear to be in financial trouble themselves. They continue to sell my product. They've paid more than ten times that amount to us over the years, albeit always late. We've point-blank asked them why, but we are either ignored or get nonsensical answers that have nothing to do with the question asked.
 
The only theory I have is that our contract grants them our source code, held in escrow in case we go out of business. If they were to force us into bankruptcy, they'd essentially get our main asset for free. 
 
Of course, that same contract also specifies 30 days to payment, so that document's already void. Regardless of their reasons, I'm confident a court would decide in our favor. That's if we could afford to sue them. They're a bigger company with deeper pockets, so they could drag it out until we ran out of money, which wouldn't take long.
 
So I need to decide if it's worth the gamble to retain a lawyer, if only to credibly threaten a lawsuit. That would force me to spend money I need to make my mortgage payments. I'd like to hear from anyone who's had a similar experience and, most important, how it turned out.
 

It never hurts to talk to a lawyer whom you trust.  Every time I move into a new town, finding a good lawyer is first on my agenda.  I'm not a criminal,  it's just always good to retain one.

Dave, I ran a signmaking company for 23 years. I've been in this situation more than once. Sadly I have to report it never went well.
 
This probably won't help...
 
The worst scenario was a similar amount, £40,000 GBP. Large contract (for us), but handled by a design/advertising agency, not the end user. Same thing, fobbed off for months with excuses, couldn't get the money out of them. Took them to court and applied a Mareva Injuction;
 
http://www.lawteacher.net/free-law-essays/english-legal-system/mareva-injunction.php
 
It's a ferocious (and very costly, to us at the time) process which effectively froze their assets and stopped them trading or making any financial transactions.
 
We were 48 hours too late, they'd drained the bank accounts, spilt into 2 new limited companies and carried right on trading. We were even anonymously sent a parcel containing all their bank statements by a well-wisher in their company, but it didn't help.
 
This is one of the reasons why I'm now a gardener in Scotland.

Yes, you need a lawyer.  Even if only to provide a credible threat letter.  Initially.  Maybe try to get a lien of some sort in place without first threatening so they aren't tipped off.

The amount of money in your bank determines the amount of justice you can buy. Big companies with deep pockets will starve you out. If I was a software developer I would install some sort of kill switch. 
 

Time to join that band.

Yes you should get an attorney and put a lien on their assets. In the event that they try to sell said assets they'll have to settle with you. You could also send a cease and desist letter on an attorney's letter head that they are violation of the contract. Just my $.02.
 
Rocky

One of the things I try to keep up with is to keep my finger on the pulse of the companies and people I deal with.  My software lets me know as soon as someone goes over 30 days, and 60, 90 are the next waypoints.  I try to get in touch with customers and find out why they are late. 
 
Some are just lazy procrastinators, others have a 45 day billing cycle, sometimes the bill gets misplaced or lost in the mail. I guess I'm lucky or blessed because after 24 years in business, I've only been cheated out of less than $1000 total.  There have been a few times where I thought there were going to be issues but a phone call to the right person and the issue was resolved and a check was cut.
 
The longer a customer is allowed to not pay, the harder it then becomes to eventually collect.  Since I don't know all the details.... if you were to stop providing your service or product to them would that make any difference to them or is it not possible to do that?  That is my leverage.... I cut off the service and lock them out until I get paid.  I've not had to do that very often. Just the threat of doing that often gets a check in my hands quickly.

If I had a customer like the one you describe, I'd probably seek the services of either a collection agency or a lawyer or some combination thereof.  They will take a sizable chunk of what they collect, and it might not be the full amount. They will very likely negotiate a settlement and they will keep 30% or more of the money they collect.  However, having part of that is better than having none of that..... and remember that if you are on speaking terms with their accounting dept, you could negotiate that kind of a deal too and keep 100% of the settled amount. It's always better to settle things like this and get some money. 

Would they suffer if you stopped doing business with them? And the flip side is that if you go after them with a collector, they will very likely not do business with you. So be prepared to lose a customer.  Taking them into court, if it becomes necessary, might, depending on the laws in the state, allow you to sue for additional damages and punitive costs as well as legal fees, in addition to and above the full amount owed. Most lawyers and collection agencies will discuss this all in detail with you for free. That way, you can make a better decision than that based on what advice you get here.

I heard this on Clark Howard's money show.... regarding being let out of or forgiven a debt or a settlement amount. If they don't pay in full, or agree to a settlement, you can always send them a 1099 form so that what they don't pay you becomes to them, taxable income.  Of course that's like burning your bridges.
 
Yeah, check with a business lawyer in your state.  Many will work on a percentage basis.

It's a pity the small claims court limit is $10,000 because that road is a lot easier. I had a similar situation albeit nowhere near on the same scale - a customer ran up a bill of $1000 and just refused to pay. No reason, no dispute, just blanked all my invoices and emails and texts and calls. This guy is a very wealthy CEO in a large financial company and had no reason not to pay what to him was peanuts. So I went through the small claims court and it was a breeze - he didn't show up to the hearing, my evidence was beyond dispute and they ruled in my favor. The trouble is not even a judgment against him was enough to induce him to pay, and he held out for another year before cutting me a check - for the amount he owed minus the expenses I was awarded, which I now plan to recover in the same way out of principle.

Some people are just trash, it's as simple as that.

Thanks for the thoughts, everyone. More to think about...
 
I have to tread lightly, because this isn't just my biggest customer, it's a company that has supplied over half of our annual income for 8+ years. Close to a million dollars altogether.
 
Cutting off our support would definitely hurt them badly, making it pretty much the thermonuclear option. And an immediate end to the relationship and any future revenue. I could end up living in a van down by the river.
 
I'm thinking now that rather than launch a scorched-earth assault I should just send them a registered letter telling them that our contract is now void due to their noncompliance. Let them read between the lines.