Hi, to solve your problem you will need to use ADB and Fastboot commands.
If your on a windows computer, let your device boot into recovery- plug it in and then hold shift and right click and in your android-sdk/platform-tools folder hit "open a command line here"
Then try fastboot reboot-bootloader
Hopefully it boots into bootloader.
Then flash a stock ROM find here:
https://developers.google.com/android/nexus/images
In the android development forums there is a thread called "How to flash back stock/and unbrick your device"
Follow the instructions there to successfully flash the stock 4.1.2 ROM.
** Please note, this will wipe all data including the virtual SD.
You may then proceed to flash CWM Recovery
fastboot flash recovery your-recovery.img
Then boot into recovery and flash the ROM you want.
Hope this helps you!