Possible solutions:
Soft reset the Nexus 7 .. by holding down the power button for 30 seconds.
Factory reset without the touchscreen .. by pressing and holding the Power button and Volume Down then press Volume Down twice and hit the Power button to select Recovery Mode.. Hold the Power button down and hit Volume Up and then use the volume rocker to select the “wipe data/factory” reset option and press the Power button to select it. Use the volume rocker again to select “Yes – erase all user data” and press the Power button to activate.
If the reset doesn’t work then you should contact the retailer who you bought the device from.