There are a few possible causes to this problem:

1. LabQuest is asleep
There is a power management feature that will turn the screen off after 5 minutes (default) of idle time. To turn the screen back on, simply tap the screen, press a key, or press the power button. If desired, you can adjust the idle time (tap Home > Control Panel > Light & Power).

2. LabQuest has crashed
An extremely low battery can cause the LabQuest software to crash while shutting down. If this happens, there is too little power to restart the unit, even when using an AC adapter, and the unit will not charge. When this happens, you will need to reset the unit.

A. Hold down the reset button on the back of the LabQuest for 15 seconds (see How do I perform a hard reset on my original LabQuest unit?).
B. If this does not work, it may be necessary to physically disconnect to battery (see How do I remove or replace an original LabQuest battery?).

Resetting the unit will force the LabQuest to completely shut down. After all lights are off, wait 60 seconds, then connect the unit to AC power. It may take as long as two minutes before the screen lights and the unit begins the 2 1/2 minute boot-up cycle.

Once the LabQuest has been reset, charge the unit for several hours before trying to use it with battery only. You will also want to make sure you are using LabQuest App version 1.5.1 or newer (Tap Home > Control Panel > System Information). If you are not, update the software. A free update is available at:
https://www.vernier.com/labquest/updates/

If you continue to have trouble, contact Vernier technical support for further assistance.