loaded video file won't play

hi,i have a e250 sansa player, i think v1, but i’m not sure.  i converted and loaded a file using the converter, but when i tried to view it on the player, i was told the file needed to be converted using the converter, which i’d already done.  HELP!!  I have been trying to get this to work for 3 days, to no avail.  thanks.