It could be a drive issue, it could be corruption in the project files or any of the media.
Check all drives to see if any of them is over 90 per cent full. If so, you must free up some space before you can proceed with any other troubleshooting.
Quickest remedy is to access your last manual backup -- please tell me you have a manual-backup routine? If not, try your last Autosave Vault backup.
If you don't have either of these -- suicidal -- then try copying the project file to a location where the media can't be accessed and see if it will open. If not, you may have to locate a 5.0.1 FCP system to test that project file.
And let's repeat one of the golden maxims of LAFCPUG: "Never upgrade in the middle of a project." Adding, "especially right before a deadline".
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