ok guys I had this issue after trying to update to 4.3. I did every other suggestion from turning it off to wiping the data. But this is how i fixed mine,
1) download nexus 7 V5.0 or newer toolkit (you need windows for this doesnt work on mac)
2) after installing nexus 7 toolkit and running it press Y to check for update
3) then pick the version of your nexus 7 (e.g. wifi or gms/hspa etc), I picked number 14 (because I have the 32gb wifi version and I pick android 4.2.1)
4)After going to the next section, Press 1 (then enter)
5) after the drivers are installed connect your nexus 7 to computer via usb cable then bring the tablet into fastboot mode (hold vol up, vol down and power buttons)
6) then press 3 in nexus 7 toolkit to unlock the bootloader
7) after entering the next section press 1 then enter to unlock the bootloader (a message will pop up on your device asking your permission to unlock the device, go ahead and accept it), then your device will reboot
8) after your device is unlocked, enter bootloader (follow step 5)
9) then press 9 and enter to download google stock rom
10) then press 1 and enter to download google stock image of your choice (I picked number 14)
11) after the download is completed go the same menu as step 9, then press 2 and enter to Flash google factory image
12) then it will ask your permission to start the procedure, then press y
13) The process will continue for a while and after it is done it will restart your tablet and it will be ready to use again