Well the simplest way would be to use the Lumia Software Recovery tool. This installs a new firmware and should fix this issue. If it doesn't then you have a hardware problem.
The LSRT will erase your whole phone so be sure to backup.
You require a PC and internet connection. Download is about 4GB.
FAQ - How can I recover/reset/restore my phone software? - Microsoft - USA
If you want you can try a soft reset to see if there that fixes anything. Also check recently installed apps, especially those that may effect the start screen.
Soft reset - Hold volume down and power button at the same time until the phone vibrates. Phone will restart.