I see a number of visitors getting here searching how to reboot the HP TouchPad, so before showing how to restart a frozen HP TouchPad that stopped responding, a graceful restart of the TouchPad in webOS can be done in Settings => Device Info => Reset Options => Restart. In CyanogenMod Android this is much easier – press and hold the power button for about 1 second and you’ll get a lot of options, including reboot, power off, switch to silent mode, or take a screen shot.
And now back to the subject: if your HP TouchPad is stuck and not reacting to anything, the only way out (other than letting its battery totally discharge in many hours) is to press and hold both power and home button for over 10 seconds until the device reboots.
If this doesn’t help, I’ve read that some people who had CyanogenMod Android installed were able to regain control after holding the two buttons pressed as long as two minutes. If that still doesn’t help, you need to wait for the battery to discharge when the tablet will shut off. Another thing worth mentioning to new owners is that the power button has to be pressed and held for 1-2 seconds to power up the tablet.
If your TouchPad keeps freezing constantly and consistently even after a reboot, there may be some software or hardware issues so your next step may be uninstalling the last app or reverting the last change before which it worked OK. If that doesn’t help, you may wipe the device to factory settings (available under Settings => Device Info => Reset Options) and reinstall/restore everything. If even that doesn’t help, webOS Doctor utility is available for download to wipe everything on the device and start from scratch (requires a USB connection to a computer).
If you are still having problems after all this, the problem may be in hardware and HP support may become your best friend. God speed!