There have been reports of LabQuest units freezing when the units are in a low battery condition and then connected to AC power. Symptoms of the freeze are:

* Screen is on but not responsive to taps (may show battery warning message)
* Blue power light is on but power button will not work.
* Hard keys are not responsive.

The fix is to do a hard reset. See How do I reboot or reset an original LabQuest?