Shimano 105s are very durable but of course they can break. I would give it a good cleaning and see if that will un-freeze it. Check to see if something is stuck between the limit screws and the derailleur body. The Low/High limit screws stops the derailleur from moving outside the largest cog and the smallest cog respectively. Check this site to see how the derailleur works.
I realized my chain may be too short, putting excess tension on the derailleur holding it back. I cut the chain and pulled it out of the pulleys. When I pushed it in toward the lower gears it would spring right back to the high gears. When I ran the chain through it, it would get hung up at about the 4th gear.
Also, I found a screw inside of the derailleur that looks like it may be a spring adjustment. Any info on this?
Is the screw you found inside the der. on the "bottom" of the inside arm/link? 1055's and 6402's have a spring tension adjustment screw that's either "on" or "off", it increases the force of the spring action but in this case it if you line up the arrows (one on the screw head, another on the link itself) that's max tension and should help push the der. to the outside. Edit - the screw doesn't have an arrow - line the slot up straight with the arrow on the body.
Dumb question but - when you disconected the cable did you pop the bolt up inside the body a bit?
That bolt has flat sides that "lock" it in position to keep it from turning when the nut is tightened. If it's riding a little high it can get caught between the body and prevent the der. from moving one way or the other. Not likely since it should be pretty obvious if that happened.