Sensor no longer picking up movement.
All other functions of the mouse work normally, it just will not pick up movement.
This happened to me last week on the same day as you posted your question, spooky!
To fix it I found this repair guide, which suggests that the gesture button can get stuck if it gets pressed... wrong.
I used this teardown guide to open the mouse up with an iOpener to soften the sticker adhesive and a metal spudger to peel it up, then broad tweezers to prevent tearing or grease from my fingers, I followed Steps 1-2. Then the first part of Step 3 to remove the black screws. I tested the button and its mechanism. Both seems to actuate fine, so I reattached it without tightening fully to give it a little wiggle room.
Once I reassembled the mouse and used the iOpener to soften the adhesive and smooth it back down the the metal spudger, it worked as good as new, if a little more crinkly on the bottom, which is undetectable in normal operation.