When I open up my sansa e250 files in my computer,

Go to your menu...settings...usb mode...and then make sure it is on mtp that fixed my problem

are you able to see all your music files in your sansa player? if yes, try to play those songs from your mp3 player. then you can format your mp3 player directly without connecting to the computer. before doing that check whether you have all the files as backup. after formatting, once again transfer all the songs, files you need. this solve your problem.