how can i fix my Sandisk Sansa mp3 player?

when i turn on my mp3 player it says there are no files but when i plug it into my computer all the songs are there but then it wont let me delete them. this all happened after i put some songs on it from bearshare, they worked for a while but then when i turned on my mp3 player it said i needed to sync it. but when i tried to my computer froze. now those songs wont even play on my computer. anybody know anything about how to fix this?