Lately (for 2 months now), I've been having issue with my FCPX. Specifically, the Preview Screen was blank / black, even when playing on parts with video / jpg media. Tried googling and do the all the steps people recommend: delete all generated files, clear caches, restart computer, run the program on another clean account, unistall and reinstall FCPX. All to no avail. Still the preview screen was stubbornly blank.
Then one day, I needed to access another mac, so I unplugged one of my monitors to work on that mac. Then I found out FCPX on my mac works fine. The Preview Screen is back!!
So apparently, for me, it's an issue of monitor. I don't know what caused it, but the solution is this:
1. unplug other monitors (use only 1 monitor -- I use an external monitor for this)
2. restart FCPX (no need to restart computer for me)
3. after the preview screen works fine, plug back other monitors (if you need to work with multiple monitors)
Writing it down now before I forget....