What would cause the hammer not to be pulled back when the cylinder is in? It became a problem completely out of nowhere. Not even been fired for 3 months. One day I picked it up, and now the hammer can't be pulled back with the cylinder in.

Sounds like some kind of debris is in the locking mechanism.

Take the cylinder out and see if everything functions, so at least you can narrow it down.

the hammer can be pulled, and fall. Is the mechanism (the bump) on the bottom (coming up from the frame) supposed to be moving, right? It's not when I pull the hammer.

(this may come as a shock to everyone, but I know NOTHING about revolvers.)

So it will go back when the cylinder is out?  maybe the alignment is off, and the pawl is not working properly.

correct, the hammer cocks back as normal with the cylinder out.

Possibly dried and coagulated lube gumming it up.

If your revolver is an imported single action, you may have the base pin,(cylinder pin), pushed in too far.
All imported revolvers must have a hammer block safety installed and some of the importers met this requirement by designing a base pin that could be pushed back and locked against the hammer.

Pull the base pin out and see if there are two grooves near the end that sticks out from the front of the frame.

If you push the Base pin in and lock it on the first groove, the hammer should cock and the cylinder should rotate.
Pushing it in farther to engage the second groove will set the hammer block, a small portion of the base pin sticks out the back of the frame and engages in a hole or notch on the front side of the hammer preventing it from cocking.HTH

base pin misalignment can affect Ruger Vaqueros also.  pull the base pin and the cylinder, replace the base pin.  see if it cycles, if it does, it's the base pin out of alignment, if it doesn't then it's likely a piece of gunk in there