Nexus 7 blank screen
The same thing had happened to my N7. Here's what worked for me.

Let your device stay in that state for as long as the battery lasts, i.e wait till the battery is dead.

After that is done, Power on the devise by Holding the two volume buttons and then connect the charger or connect to a computer.

Use your volume buttons to navigate to START and click on the power button

That should bring it back to life

Edit Here is a photo of the boot loader I found on Google which shows the Start