Hi [oregongirl],
First off, replace the set screw you removed from the gear. Taking it out and then trying to force the gear to move will only create a bigger problem.
A machine will "lock up" for one of three reasons. In no particular order,
1) Rust.
2) A "thread lock," in other words a small piece of thread has become wedged between two moving parts, jamming up the whole works.
3) The oil and grease used to lubricate the machine have gummed up over time into a sticky consistency.
In my opinion #3 is the most common. First rule out #1 and #2 by closely inspecting your machine, particularly the underside, with a flashlight.
If it's #3, the good news is it's easy to fix. With the last 15-91 I fixed that was frozen solid, I simply put the machine on its side so I could access the undercarriage. Then, using a Q-tip, I dabbed a few drops of rubbing alcohol on every part of the undercarriage where two pieces of metal touched one another. Rubbing alcohol penetrates into gummed-up oil and breaks it up, and best of all it evaporates completely, so it won't rust your machine.
After just a few minutes of letting the alcohol penetrate, I was able to rotate the handwheel slightly, but it was still hard. But this at least let me see where the *moving* parts of metal were on the undercarriage, and then I applied more rubbing alcohol at those points to break up the unseen gunk holding them together on the inside. Then, more turning of the handwheel to let the alcohol penetrate more.
Once all the gunk is gone and your machine turns over smoothly, you then want to oil all the moving parts down there.
Two tips if going this route: 1) Lay a towel down under the machine, and 2) be very cautious not to let the rubbing alcohol spill onto any painted part of your machine, or it will eat through the finish.