How can I restore missing Video thumbnails? (Windows 7)

[Supreme101]

My thumbnails usually have a "screen cap" of what the video is. Now, it just look like this (see attachment below). It's only with my videos that I'm having this problem with. And, it's not just "some" videos that doesn't have a thumbnail, it's all of them. What can I do to fix this?

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[Supreme101]

Didn't mean to make a second post. I'm new here :/