Nexus 7 is not booting up after android marshmallow update
Download the Nexus Root Toolkit from here.  It has options to restore any Nexus device back to its factory settings, including bricked or boot looping devices, under the Flash Stock + Unroot button.  You should select the option Current status of Soft-Bricked / Bootloop.  The toolkit will walk you through, step by step, restoring your Nexus to working condition.

You may lose any data stored on the device.