Problem:

Revolve project - open - could not be loaded.  OK until yesterday.  Nothing has changed.

Resolution:

Reviewed contents of the project folder, looking for RP1 and RP3 files.  Traditionally the RP1 file is about 3x the size of the RP3 file, these files must be present for the project to be opened.  If the RP1 or RP3 files are missing, finding them in a previous back up and opening the project should restore any problems.  Upon closing project, verify that the RP1 and RP3 files still exist.   

Also verify that there aren't any .lck files.