I'm extremely upset and I'm on the verge of smashing my fucking computer monitor because I worked for 3 hours on a song for part of a videogame soundtrack I've been producing for a friend of mine on LMMS. Now, I'm probably not gonna be able to create the soundtrack and I'm probably never gonna use LMMS ever again if I can't recover my hard work. Is there any way to recover it?
Yes. Go into your LMMS folder to "Projects" and open the .bak file that has the same name as your project. It's a backup file that hopefully has most of your progress. And of course, I implore you to save often (or turn on autosave).
Also, although three hours is a considerable amount of time lost, it doesn't have to kill your track. It's understandable to be angry after you lost all of that progress, but since you've already done it once, you can probably recreate what you've already done in significantly less time.