I’m frustrated

Thumbnails turned into black and white and won't play

I have Nero MediaHome installed on my pc as standard,everything has been fine and I was going to purchase an upgrade, when a free upgrade appeared on the pc, which I downloaded.This 'new and improved' version worked fine for a day then it changed some of my video download thumbnails into black and white,with a red triangle in the top left corner and it will no longer play these videos.Although the colour thumbnails do appear elsewhere in Nero, but I cannot do anything with them.
Can you please supply an answer on how to solve the problem or have I lost the videos completely.I am not very good with computers so any help will be very much appreciated.Many thanks,
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