Well, @Ajay I am afraid over here but this gives me a feel that you have messed up with your device may be you should try performing a Hard reset via recovery menu (It will wipe all user data and firmware settings, but yes contents over your SD card will remain intact)
Steps to Hard Reset Nexus7:
- Turn the tablet off by pressing power button.
- Once the tablet is switched off, Press and hold the Power key and the Volume Down key.
- Now over the menu use the volume button to scroll down to Recovery.
- Now Select it by pressing the power key.
- You'll see the Android pop up, then while holding down Power, press Volume Up.
- Use the volume keys to scroll to "Wipe Data/Factory Reset" and press Power to select it.
- Scroll down to "Yes - Erase All User Data" and press Power to select it.
All done, now Hard Resetting is complete !! Wait for the device to Reboot check if now you are able yo use your device.