You did not specify which model Sansa you have, so it is difficult to give you more than general information.
You might want to check the
SansaE200Unbrick wiki page. The page is for e200 and c200 (v1).
If you have an e200 (v1), you may need to reseat the memory module, as BlastTyrant suggested. It can cause problems if the e200 is jarred or dropped. To reseat the memory module, remove the back cover and press the rectangular black foam above the battery. The memory module is under the foam.