Hard to say without specifics, which media player, what was the update? Codecs?
I personally use Media Player Classic and the K-Lite codec pack to play all videos. But many will recomment VLC as it supports just about all video types nativey without any additional codecs.
If you just recently did this, I'd use a system restore point from before it started and stick with the older version. You can always try again when a newer version comes out. Codecs are the likely cause, that's why I asked about them specifically.