Have you tried taking off the top cover and anything else that comes off easily? I'm not familiar with that particular machine but have taken apart and fixed many a thrift shop mechanical machine in my day. The covers with screws holding them in place are very easy to remove. Occasionally I ran across a latching mechanism and couldn't figure out how to get the cover off; in that case I would have my mechanically-inclined husband take a look. He was always able to figure out how to get the cover off.
That seems always to be the first step. Then I would turn the wheel by hand, change knob positions, etc. while watching the "innards" work. I could usually spot the problem area that way.
My best guess is that there is a plastic part inside that got broken. There is no easy fix for that. Your best bet would be to find an older sewing machine repair guy (or shop owner) and take the machine in to explain what happened to it. I used to donate the thrift machines I fixed up. On one machine the foot broke while I was cleaning it. Took the broken foot to an old shop owner and explained I needed a replacement. He went into the back of the shop, came out with a foot he had scavenged from a non-working machine, and didn't charge me anything.