I've been using Movie Maker 15 Trial for days now, and exporting always worked. Just tonight, when I tried to do the same thing I always did to export (clicked the export button in the top right), it's grayed out. The Export sub-menu on the File menu has everything except Upload to Youtube grayed out.

Why can't I export all of a sudden?

*** NOTE: My trial was not expired. It is now. I will gladly purchase the program, but I need to know that this will not continue to be a problem for me when I do.