windows media player 11 not playing music files

I had to transfer my music files from another laptop to my Vista Ultimate
laptop because of a crash. But the Sony Vaio laptop won't open/play the
music on windows media player that I have (and it did before). Music will
play on my Creative MediaSource player that came with my Zen player.
Obviously this has something to do with the recent addition of my music files
from another laptop. How do I get the music to play (and, yes, all paid
for!) now on the wmp 11?