What have you done so far to try and correct the problem?
If nothing, I would start by checking the volume of the phone. Then, go into Settings > Sounds if the volume isn't the issue. Start by checking to see if the volume levels for ringtones and other media are up. Then, check to make sure you didn't accidently move or delete the file that is being used as your Notification ringtone (check this by switching to a different sound, see if it plays, then switch back).
After those steps, let us know if it helped or if you need further aid.