Phone automatically restarts. Has been dropped like 1 month before.

The phone is automatically restarting. Previously when it had been dropped at the moment the speaker was buzzing and the bottom half of the screen went grey. Also at that moment it wasn't responding and i had to switch it off by pressing the vol up+power combination . After that the phone booted up normally and was working but the speaker was less louder than before. Later on after a month or so the phone automatically restarted . The reboots only happen occasionally. Pls help. Any idea as to why this is happening.