For that download the official Google now launcher.(app by Google itself).your phone will start to look like a nexus.
Tap and hold on the home screen.then click on settings.there will be the option in one of the menus
It happened to me also.
First copy all the files like music,books and other that you need.
Then go to settings and factory reset your phone.
After that switch it on and you will get a screen telling you to choose language,wifi connection and then it will tell you to login with google...
Google how to open boot loader.Go to recovery there and delete your dalvik cache and cache partition and restart.
If it doesn't work post back with detail on what you did with the phone(did u try rooting or anything beacause it happened to me).