Yesterday, my right hand, rear shifter crapped out. I went to the LBS , and I dislodged a small black piece of plastic. That explained why the shifter wasn't clicking.
I got a new Shimano shifter, and I installed it in five minutes.
I suspect the extreme cold weather made the plastic snap. It was around 15 degrees F.
I paid $20.00 for the new shifter.
Photo of bike.