The unit may have the problem that befalls the Palm Pilots where something gets under the escutcheon and presses on the touch screen at the edge which overrides any other inputs.

Another possibility is a stuck button. The buttons are usually scanned as a matrix and at startup if a button is stuck that can make unit unresponsive.

Without a logic analyzer and the firmware listing you likely would not get far if one of the logic chips has gone bad unless it can be found using freeze mist and heat gun..

Stuck buttons or touch screen is usually easily found without schematic.