Take lockdown bar off, slide glass of and set some where safe. Lift playfield up and see where balls roll out of or make noise.

If stuck under a plastic or something. Free them. If they don't kick out of something. Then it's time to pull fuses and check with a meter to see if you blew one.

LTG : )

Medusa is a single-ball game. (only one is installed)
So I would assume you're seeing the ball sitting in the outhole. And it's not kicking the ball over to the shooter lane, at start of game???
1) Dirty/misaligned switch in outhole.
2) Bad coil for outlane.
3) Bad transistor that powers the outlane coil (or it's controlling IC).

If you don't already have the manual, go get it at www.ipdb.org and follow the test diagnostics to test the switch and the coil.

You should be able to clean/adjust is a switch maybe. Coils & board work is more involved. But you should have plenty of pinball folks in your area that can help too.

Good luck.

Medusa only has one ball

Then check fuses first.

LTG : )

Ahh.. yeah, good catch. I was kinda of thinking the other coils were firing (if the ball was manually tossed around the playfield).
Again... test mode would indicate if no coils were firing; and therefore check fuse(s)?