My 250M has frozen

I recently bought a Sansa M250 (about 6 weeks ago) serial #BE0611NA043571 the system was working okay although it froze on a couple of occasions which required me to remove the battery to unfreeze it.

Yesterday I loaded up some more music to the system and know it does not start up every time I turn on the system it starts up then freeze at the start up screen displaying the normal start up screen:- SANSA V4.0.44a
                         M250 2.0GB

The only way to turn it off is to hold the on/off buttun for about 10-30 sec or remove the battery, I have also changed the battery to no effect and when I plug into my computer the computer does not acknoldge its there, Why is this happening what can I do to fix it?