Use the following steps to sideload a rom:
1. Head over to TWRP and perform a data/cache/dalvic wipe.
2. Copy the rom.zip and gapps.zip to the fastboot folder in your computer.
3. In the recovery, go to advanced and select adb sideload and swipe to the right.
4. Now open cmd window from the fastboot folder and run the following:
adb sideload rom.zip
5. After it's done do the same with gapps.zip.
If you don't want to use a custom rom then you can flash the factory images from here : https://developers.google.com/androi...mmerheadkot49h
For the steps, follow this guide: http://forum.xda-developers.com/goog...image-t2513701