The SD drive in the unit has failed. To verify this, press down the Power and volume up keys until it powers on (have the unit plugged into the charger in case the battery also doesn't have a charge on it). It will boot and fail with a message about the SD card failure. If it can read the SD card, it will reinitialize the unit just like it was when you first bought it. Warning--this will delete all files you have created and erase all games you added.