Get a manual. IPDB has one: https://www.ipdb.org/machine.cgi?id=1382
Pinwiki is good for troubleshooting: http://www.pinwiki.com/wiki/index.php?title=Bally/Stern
There should be a master volume inside the coin door. Is this turned all the way down?
Pretend you are a quarter. Open the coin door. Try activating the switch a quarter would activate if it fell into the coin box. Did that add any credits?
Look at page 10 of the .pdf (page five in the manual itself). That shows how to set the dip switches for up to fifteen credits per coin.
Make sure there is a ball in the game.