I was in the middle of a project, in X3 Producer, I don't know what I did but all of the sudden the the master bus went silent. It's not muted, I was able to route my other busses direct to my Scarlett but it's obviously not the same. Is there a way to drop a new master into the project? Is there any idea of anything to check to try to remedy the bus issue?

check your routing on your 18I8. this happens to me every once in a while if I bump a fader on my control surface.
it levels out (turns the hardware fader output all the way down) thus not allowing any signal to pass through.
this drove me crazy trying to figure out what it was. might not be your issue but its worth checking.
 
just look in console view at your hardware's output (not the master bus) the hardware output.
PS- its all the way to the right / sometimes either hidden or half hidden, you need to drag it out to see all the hardware's outputs.

Do you have any plugins on the master channel?  If so, try disabling them one at a time.  Could be a plugin issue.  If nothing else, delete the Master.  Install a new buss, label it "Master" and route it direct to your hardware.

Ok. So there's not a specific Buss for the "master"? I'll try this tonight when I finally get home.

Ya, that took me a few years to figure out. When you open a midi file you never get a master buss. I hated this. Then I tried it one day and it works. You can insert busses till the cows come home , name them anything you like and they serve any purpose you desire if you send things to them or insert things in the bins. Then you can also send the output to any of the choices, your interface or another buss. They are basically universal. Even the metronome is just any buss you send the metronome to. You could call it  " School Buss" and it will still be the metronome.

stm113cw
Ok. So there's not a specific Buss for the "master"? I'll try this tonight when I finally get home.

the master bus is the sum of all tracks and other busses.
your master bus is routed to your hardware bus (the Focusrite 18I8)

Ok, I will do that. One thing I noticed recently but I need to check for accuracy it that the compressor type on the PC of the master is the type that allows for high pass but not on the busses which is what led me to wonder if there is anything special about the specific master Buss. As far as routing, plug ins, Pro-channel, how tracks are summed or anything of that sort.

The Master Buss is just like any other buss - except it's the last one in the chain before it hits your mains/interface/soundcard.
 
It has exactly the same characteristics as all other busses and access to exactly the same plugins.
 
ALL other busses MUST have their output directed to the Master Buss, this is assuming that all of your tracks have their output directed to a specific buss - drums, vocals, guitars, strings, reverb, delay etc.
 
If you don't have any of these "sub-buses" inserted then ALL of the individual Track Outputs MUST have their output directed to the Master Buss.
 
Muting the Main Outputs should always result in perfect silence.