Zep, your machine needs a couple things to start a game.

Credits- could be added by dropping coins, set on free play somehow, a custom solution using a button, or opening the door and manually moving the coin switch.

Start button- if this isn’t working it could have a wire that fell off, a contact that broke off, a leaf that broke or bent.

Can you post a photo that shows credits on the machine?
Do you know how to lift the playfield?

This Pinwiki article details how to perform a switch test: https://www.pinwiki.com/wiki/index.php/Bally/Stern#Switch_problems
Your manual should have switch numbers.