Connect the LabQuest to AC power in case the battery level is very low. Confirm that the AC outlet you use is live. Let the unit charge for a minute or so, and leave the AC adapter connected. Press the power button. This will usually revive an unresponsive LabQuest. If that was the problem, let the unit charge for eight hours or more to fully restore the battery. There is no harm in leaving the unit charging indefinitely.
There are a few other things that could cause a LabQuest to stop responding. In most cases, LabQuest is a lot like a computer and simply rebooting the device will solve the problem. If the screen is black and the power button does not work, the only option is to do a hard reset. See: TIL 1777: How do I reboot or reset a LabQuest?