Why can't I open mp4 files anymore?

[melominermega2]

I used to be able to view my clips and such, but suddenly I've been hit with this message upon double clicking to open them or just pressing open on them through the file explorer. This happens with EVERY mp4 file I have and try to open now and even happens with newer ones I've downloaded after the problem.However I can still view them if I use "open with" and select a media source like windows media player or photos

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