Missing tracks!

My computer is driving me mad, Im losing random tracks from albums, from the
media player 11. Its very strange and Ive searched my hard-drive with no
luck. Can any bright soul help and give me any ideas where they might be- Ive
lost so many songs and certainly havent deleted them, its a pain to have to
rip CDs all over again just for the sake of one or two songs missing...from
each album! Please any ideas greatly received!!
Amy