All the shifting was working smoothly until my bike fell off to the ground (edit1: to my right side hitting the right side of the bike) with me couple days back. Now after about a week when I took my bike for a ride I found that the rear shifter doesn't work as intended.
The problem is when I push the lever (rear) to shift to a smaller gear (larger cog), the lever doesn't come back to the neutral position, thus I have to manually pull it back to the neutral position. When I push the lever, it sort of gets stuck there. So when I want to shift to higher gears I have to push the lever then take it back and then push it again and so on and so forth. Shifting to higher gears works perfectly and generally there's no problem with indexing or any other tunning thing, only thing is the lever getting stuck. Can't be a problem with cable either.
Front shifting working as usual. Today I opened the case just to see if I can find the reason for the issue by visual inspection but I can't identify anything to fix (have attached a pic after removing the rear shifter case).
Any help towards fixing this is highly appreciated!
bike: specialized sirrus, gears: shimano alivio 8 speed
EDIT2: some other pics of the rear derailler (just for you to see if it's bent or not)
EDIT3 (closure): First I tried losing the cable tension to see if the lever is still stiff and it was. Which made it clear that the problem is with the shifter and not with the cable. And then I removed the back cover of the shifter and voila! it started working. But it gets stuck when I fully screw the nut in with the back cover. So I just screw it in to the extent that the lever is working and left it there. 
