12.04 update breaks video

[rikona]

Just updated 12.04 and video is broken. Neither VLC, Movie Player, or
Dragon player will show the video part of MP4, avi, or wmv files.
Dozens of MP4s that used to play just fine are all broken. Trying to
play these in Ffox gives "file is corrupted". A 14.04 box, recently
updated, plays MP4s just fine. The audio part seems to be fine - just
a black screen instead of video. . How can I find out what happened
and fix it?
  
All suggestions much appreciated,

 rikona