Windows movie Maker was working, now shows little triangles when trying to work on viedo. windows 10 and paid version

Hi there, i bought Windows movie maker to make videos for youtube.  I was using it last week just perfectly then out of no where all my videos have the little triangles on them.  It happened when i saved a video and opened another. when i went back to the original video nothing would load.  This is really frustrating because i had spent a good 3 hours editing just to have it all stop working. it's even more frustrating because i paid for it and can not find a way to fix it.  I really just need some info on what to do. I've already uninstalled and reinstalled several times and tried running as administrator.  Nothing is working and i always get the little triangles when i try to open any video now.  Please help! Or i guess i'll have to try and look up a way to get a refund for a defective program.

Thanks