My back wheel will no longer spin freely
I am pretty sure the issue comes from your disc brake.
Check the brake pads first : there must be something wrong here because you say it was worse after you removed and put the wheel back.

I guess your brakes are hydraulic : sometimes when you store your bike in a humid place for a long time, the braking fluid will kind of "dilate", resulting in less space between the two pads, hence rubbing on the disk even if you don't press the lever.

Another test you could do is removing the caliper : if the wheel spins freely, that confirms my theory.

In that case, you need to push the pistons back to their initial position (using something to pry them apart, without damaging the pads). If you cannot, you might need to remove some fluid from the hose (there should be a small screw at the lever), but be careful not let some air go in the hose.