Why can't LabQuest power back up after using an SD card?

This information pertains to the original LabQuest.
If you have LabQuest 2, see LabQuest 2 FAQs and Troubleshooting

If an SD card is accessed and then removed from the LabQuest before powering down, the LabQuest power button LED remains lighted. It is not possible to power it back up. Pressing the reset button on the back of the LabQuest is the only way to restart it.

This problem has been fixed in LabQuest App version 1.4 and newer, which is available as a free download at:
http://www.vernier.com/labquest/updates/