if you mean you cant view them on websites it could be your browser (if your using ie its definrtly your browser), if you mean videos youve downloaded its possibly your codecs, if your using windows media player download a codec pack
Download K-Lite Codec Pack 8.40 (Full) - FileHippo.com
or better still vlc
Download VLC Media Player 2.0.0 - FileHippo.com but its strange that its stopped working if its worked in the past, you might have changed a setting or something youve downloaded messed it up, other possible cause is malware start doing some scans, start with malwarebytes
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