Tried to replace a background and now all of my work appears to be gone

Have just spent 2 days working on a movie. The movie is made up of photos, title slides, audio and backgrounds (no video)

I tried to replace a black background with another colour (hit 'replace from start') and now nothing is working. All of the copy I had put on the title pages has gone. When I click on anything (photo or title) it just shows black in the preview screen. Also when I hit the space bar nothing happens - i.e. doesn't play.

I tried hitting command Z to undo a whole load of things but still nothing.

This movie is for a big charity ball this weekend and I can't bear the thought of starting over. Please tell me this is an easy fix!!

I am a very novice user so hoping that's the case.