I have something very similar happen to me. My file is still a Mathcad XML Document. Same warning showed up when I tried to save the file. The Size is 0 KB now though. Is there any way I can open the file even in Word?
If your file size is 0, then your work is lost. No hope of retreiving any information from it.
I hope (for you) you have a backup, or any other form of previously saved version.
I do have a format version we used. It seems I lost a few days work though. Are backups done automatically or you have to create them so often?
In mathcad 11 there isn't any backup function, but then Mathcad 11 hardly ever ruins a mathcad sheet file...
In Mathcad 15 I believe there is an AutoSave function, but it will save to the same file. If Mathcad 15 goes bezirk, it might save your file as a null file... That's how you got yours?
In Mathcad Prime there's no autosave, and I've seen some reports of users having corrupted sheets....
Better safe than sorry: Be sure to regularly save your important work in a NEW file (append the date and/or version number to the file name e.g.).