Reset FCP preferences to see if that helps. Hold Option and Command while launching FCP and choose Delete Preferences. It will open the default Untitled library. Re-open the library you were working on, set the Preferences back to the way you like them (disable background rendering, reset Import to Leave Files in Place, etc.) and see if the issue has been resolved.
It might also be an incompatible audio plugin. Temporary disable all plugins by renaming /Library/Audio/Plug-Ins to something like Plug-Ins.disabled.
If FCP still won't launch, post an https://etrecheck.com report here by pasting it into your reply using the 'Additional text' button below.