Electronic door bell not work?

I have an Electronc doorbel for front and back door, front chimes twice back once. Since someone tried to deliver something and sat on the bell, it buzzes constantly, as a result only way to stop this was to unscrew slightly the terminal. Now when i try and tighten it just buzzes and will not dong either bell, tried everything when tight just buzzes and no chime, only way to stop it is to untighten the screw.

Mark
July 2010