First, make sure that your LabQuest 2 is really frozen, it may just be that the screen calibration is off, or that some parts of the screen are not responding.
TIL 2766: LabQuest 2 screen is unresponsive or acts like I am tapping on the wrong spot.
If holding down the Home button for 5 seconds doesn't do anything, hold down the power button (on the top of the unit) for 8 seconds, and the unit will reboot. Then check your software version; tap on "Home", choose "System", and then "System Info."
LabQuest versions 2.7.0-2.7.2 will freeze if left turned on for ~5 days and not connected to Wi-Fi. This is fixed in newer versions: http://www.vernier.com/support/updates/labquest/labq2/
LabQuest versions 2.2.0 or 2.2.1 will freeze when connected to certain Wi-Fi networks. This is fixed in newer versions: http://www.vernier.com/support/updates/labquest/labq2/. Note: If you are having this issue, turn off Wi-Fi before you update, (if it freezes in the middle of the update, you will need to contact us). After the update has been installed you can turn WiFi back on.