Unplug the unit.
Get a toothpick and insert in the AV Port until you feel a click (uSwitch at back of port)
Hold switch in and plug power in to the box.
Keep holding in switch until graphics appear on screen, then release.
Unit should factory reboot.
If not, you will need an SD Card type Firmware flashing file set, and it usually has to be explicitally for your Build #.