I'm running Ubuntu on my Nexus 7 and it appears that the device locked up. Is there a procedure to trigger a shutdown or hard reset??
If the device is locked up and will not respond to input the first thing to try is holding down the power button for approximately 20 seconds, this will power cycle the device. If this doesn't work then perform a hard reset, press and hold the volume down button, and then press and hold the power button until the device is reset, showing the Google logo, indicating the bootloader is up.