I'm writing this in order to help others. I solved the problem. There are two separate files for each project you create: (your project).mpdp and (your project).Mpdp Project Data. The project Data file can become corrupted for any number of reasons. Computer updates, an electrical surge or blackout, you get the picture. If this happens, you must restore your project Data file by either downloading from a cloud back up or a back up hard disk. Then you're good to go. Hope this helps.