I ran into the same problem this morning and after searching and scratching my head for a few hours, I trashed my preferences, restarted the machine, tested FCP and created a new user on my system. Nothing worked.
***Fortunately, before I emptied the TRASH when I trashed my prefs, I took the last RENDER FILE out of the trash and placed it on my desktop.***
I emptied the trash and tried a bunch of things until I got an error code -600 (serial number/program is not recognized by Apple)(ie: cracked software). I turned off my airport, opened FCP from the render file icon on my desktop and it worked!
I had to reconnect all my files, then I copied my BIN, I opened a new project and pasted the BIN in there, I created a new sequence and copied my old one into the new one and, finally, I backed up the new project to a different external drive.
The only thing missing from my project was a few clips from before my last rendering on the original project but that took only a few minutes to fix.
Hope this help, good luck and, trust me, from now on I’ll back up my projects twice a day!