Navigation Bar suddenly disappeared on Nexus

This can happen on any phone with soft navigation keys.

Yesterday, I was tweaking my Nexus 4 after updating to Lollipop. When I restarted my phone I found that the Navigation Bar had disappeared! Even to switch from one app to another I had to restart my phone! Then I found a way to fix it.

Install Root explorer or ES Explorer. Open it and go to /system and mount it in R/W.

Then open the file build.prop in the current folder using text editor.

Then go to bottom of the file and change the value of "qemu.hw.mainkeys" from 1 to 0

Reboot and you are done!