In the meantime, try a little WD-40 or similar on all the joints.
Take the screen out to access them.
Take your handle off.
Squirt some into the gearbox from both where the rod goes in and the handle goes in. Also try to squirt some between the gearbox and the window frame.
Some operators contact the window frame where you can't see it.
Put some vise grips on the handle portion, spray some more into the gearbox, gently wiggle the gears from the handle. Repeat. The gears should slowly start to loosen up.
When they do, try to get someone to help move the windows as you turn.
With patience and persistence you may get your windows working again. The key is slow and easy or you may break something. I've gotten a few working again following this technique.
If you do, as others have said, it is a cheap part. I'm doing mine and Randy Bishop says you can pop rivet them in from the outside of the frame. I posted a sketch of what he described.
I have one of my windows out now. I want to try using Jig-a-loo Nano as a lubricant. I tried the graphite version and it makes a black mess of everything. Nano lubricant | Jig-A-Loo