I just went through this very painful exercise.
30 minute film was in jeopardy for a couple of weeks but now it is back on track through much trial and error.
More details are needed, but I would suggest getting in touch with Apple.
What version
What codecs
What effects
How long is the project?
Have you backed up the old versions folder?
Are there any files in the back up folder?
David
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The shortest answer is doing.
Lord Herbert
https://www.davidbattistella.com