The Nexus 7 stock bootloader 3.34 has a couple bugs:
1. When booting to fastboot with any sort of USB plugged in, your buttons on the side may not respond. In this case, pull the cable and hold down power + Vol Down until it reboots. The buttons should now respond and you can plug USB back in if you wish.
2. The bootloader has problems booting to recovery. It might hang at the Google screen. In this case, get back into the bootloader, connect to a computer and
> fastboot flash recovery recovery-clockwork-touch-188.8.131.52-grouper.img
then use the buttons to boot into recovery. Don't worry, your nandroids are safe.
Why does this stuff happen? Called the Google Nexus Support line and they're aware of the bug in the bootloader. Until this is fixed, don't flash strange ROMs without a computer nearby to get you out of a soft-brick situation.
BE AWARE that if you flash back to stock from the Google image after getting back into recovery, the next time you boot up, your recovery will be GONE. Try to flash a stock image from XDA instead that doesn't overwrite the recovery and save yourself a headache.
That is all.