Originally Posted by cyccommute

The broken cable problem is only really a problem with 10 speed. Your issue is with the lubrication inside the shifter and is a common problem with Shimano shifters. Some WD-40 sprayed into the guts through the cable port will usually get the lubricant refreshed and the shifter working again.

You’ll need to run the shifter through several cycles. It wouldn’t be a bad idea to change cables as well. All that takes is to release the anchor bolt on the derailer, release the shifter as much as possible and push the cable out of the shifter while holding the brake lever partially closed. You may have to tug on the cable while pressing the release lever to get the internals to release to the point where you can see the head. Spraying in the WD-40 and cycling the shifter will help