If your m240 connects to the PC (sometimes they will in this state), try formatting it and maybe reinstalling the firmware. Invalid MP3’s can cause this to happen. Removing the batteries and holding the power button down for a few seconds sometimes resets the unit. If that doesn’t get it and it’s under warranty, it’s replacement time. If it’s not under warranty, you may be able to get it going by removing and replacing the memory card. See the following forum thread for this fix.
http://forums.sandisk.com/sansa/board/message?board.id=m200&message.id=1543&query.id=108496#M1543
If you can get it going, here is a link with step by step instructions for a “complete” format.
http://www.tnni.net/~walterjmoore/m250/formatting.htm
Lotsa Luck,
