There is a way to get your original factory image back on your Device.
Googles Factory Images for Nexus Devices
This page contains binary image files that are provided for use in restoring your Nexus device's original factory firmware. These files are for use only on your personal Nexus devices and may not be disassembled, decompiled, reverse engineered, modified or redistributed by you or used in any way except as specifically set forth in the license terms that came with your device.The site contains Images for following devices:
- Nexus 10
- Nexus 7 (WiFi or UMTS)
- Nexus Q
- Galaxy Nexus
- Nexus S
So if you wanna downgrade or reset to default you can visit this site and download your binaries.