I did the same thing to my 7 the second day after I bought it. What you described is exactly what I had experienced. This is what I did to restore my 7. I kept holding the power button until it turned off. Afterwards, I charged it for a while cause the battery was low.Then I got the tablet to go into fastboot mode by holding volume down 1st and then the power button. After that, I flashed using the Nexus 7 toolkit. Upon flashing, download 4.1.2 firmware using the toolkit and flash that to your tablet. 4.2.1wouldn't work for me. That brought my 7 back to life in new stock form.
In conclusion, I re-rooted using the toolkit and flashed a custom 4.2.1 rom using TWRP recovery.
If you can get your 7 into fastboot and your computer still will not recognize your 7, reinstall the drivers to your computer. I used PDA drivers. That worked for me.
I hope this works for you.