Switch on the phone and try to get to its "desktop" screen. Start the dialer if you can and type *#7370# followed by the security code of Nokia Asha 305. The code is 12345. If the first code doesn't work, try dialing ***#7380#**.
If the method above doesn't work, you might need to visit a repair shop to get it fixed. If your warranty is not expired, you may ask for a free repair or a replacement.