I'm guessing here, but it's probably similar to iTunes and you need to rebuild your library. Just delete your existing library and then import a new one by pointing WMP to the folder where your mp3's are. Again I'm not sure since I don't use WMP.
"Whatever dramas are going on in my life, I always find that place inside my head where I see myself as the cleanest, tallest, strongest, wisest person that I can be." -Layne Staley
"The earth we abuse and the living things we kill will, in the end, take their revenge; for in exploiting their presence we are diminishing our future." -Marya Mannes
"Those who believe absurdities will commit atrocities." -Voltaire