If the above dose not work you can try a full reset this will however wipe the phone and put it back to Original Factory settings meaning you loose everything! To do this you have to hold down the Volume down, Power button and the Camera button, when the phone vibrates release the Power button and keep holding the Volume down and Camera buttons for approx 5 more seconds and you should be good to go.
If this still dose not work you will need to drop in to Nokia Repair and get them to sort it for you.