I was working on a rather large project in Adobe After effects (i spent 5 hours on it) The next day i went to reopen the file to make a few minor adjustments this error appeared :
What version of AE are you using? Exact version number please (10.0.2? 11.0.3? 12.1.0?)
What happens if you start a new project and import that project into the new one?
Also, 5 hours isn't that long.
Maybe the file is corrupt... check the After Effects Auto-Save folder for backups.