One of the great free plugins in a past was a IR loader Poulin LeCab2. The creator makes also incredible free amp simulations. Poulin LeCab2 is all over most of my old projects. I am trying to open old project and without any warning and any obvious reason Sonar crashes, screen whited out and my hair are getting whited out too.
 
Couple of hours later it turns out:  Poulin LeCab2 hates IR files to be moved to other directories. It will not warn you "IR file missing" it will crash Sonar at launch. So if you can't open old project see if you were using Poulin LeCab2 and if you by any chance messed with IR directories.

You can try opening in safe mode - and re-load Le Cab2. It might re-save with everything happy. This had happened to a few IK loaded projects for me in the past.

That's one of the little details that distinguishes free software from commercial products: how it handles unexpected situations such as the previously-specified IR not being found. It's possible that there's a "file not found" dialog in the background. You might be able to hit ENTER and get past it.
 
IF that doesn't work, follow rbh's advice above: load the project in safe mode (hold the SHIFT key down while starting), delete the plugin and save it. Then re-open it and put the plugin back in.

Yeah, thanks, that's what I did. Also putting back all relevant IR files back to where they once been helps too :-)  Currently I use Rosen Pulse for IR loader, another excellent free plugin.