LabQuest does not turn on. Both the screen and power light are black.

In most cases, this means that the LabQuest battery is fully discharged. This can happen if the battery is used heavily, or if the LabQuest is stored for a long period of time. When the LabQuest is "off", it is actually in a deep sleep state, and the battery does slowly drain.
      See TIL 2018: What is the best way to store LabQuest units over summer vacation?

First, check to make sure that the charger is working properly by connecting it to a different LabQuest. Check the power outlet to make sure it is supplying power.

To start the LabQuest again, do the following:
1. Connect the LabQuest to AC power.
2. Wait at least 5 minutes.
3. Press the power button, and the LabQuest will start.
4. Note that since the LabQuest was completely off, it will take about 2 1/2 minutes for the LabQuest to boot.

If the LabQuest does not start after charging, you may need to remove the battery to reset the battery charging system. See:
TIL 2189: How do I remove or replace a LabQuest battery?

See also:
     TIL 2077: LabQuest Battery Troubleshooting and FAQs