Sorry to tell you that this is an 'all-round' error message which can be due to multiple reasons. There could indeed be lck files, because the former session was not closed correctly. I never understood the reference to an e-mail unless it meant that cptx is a zipped folder which could have been damaged that way.
Do you have a backup file? In the most recent releases it is set up automatically in Preferences. If yes, can you open the backup file?
I hope you don't have a roaming profile, which means that your documents are on a networked drive?
Be careful with clearing the cache because it includes the cache of this project as well which could be a way to recover an older backup.
You can try to clear the Preferences.
I really hope that the project is not corrupted (damaged).