Save the file as a PSD, quit Photoshop, then re-open and try again. Do you still see the same problem?
Look in the application event log for your system to see if an error has been logged there that gives further info.
If these things don't help narrow things down, please take a screenshot of your session when you see the error, save as a JPEG, and post it here. Perhaps someone will be able to spot something else wrong.