I was changing a tire tonight, and I think I may have done something bad to my rear derailleur. I had the wheel partway back on, and I realized I hadn't shifted into the smallest cog, so I tried to do so. The derailleur didn't move, so I tried shifting to a bigger cog instead of a smaller one. The shift lever gave some resistance when I tried to go that way, so (here comes the Bad Move) I kinda forced it. A couple of times. Nothing bad seemed to happen, so I finished putting the wheel on.
I was testing the shifting afterwards, and things were quite messed up. I couldn't reach the smallest cogs, nor could I reach the largest. The shifter clicks, and pays out or pulls in cable for each shift, but the derailleur does not move as it should. Towards the smaller cogs, the cable goes completely slack, yet the derailleur does not move any further outward. Towards the larger ones, the cable is taut, but I still cannot shift into the largest cog unless I pull on the cable by hand to increase the tension even more.
I've tried playing around with the cable tension adjustment, but the result is that I can make the biggest cogs accessible by increasing tension, but no amount of loosening will let me reach the smallest ones.
So: what's my next step? Is there something I could have overtightened and stripped or bent when I forced the downshift? What should I look for?
The bike is a Specialized Sirrus, the rear derailleur is Sora, with trigger shifters.