magic mouse stopped scrolling

No change of software or hardware, no other bluetooth devices around, scrolling in any direction stopped working. Things I've tried: checked settings for mouse, rebooted, unplugged for 15 sec, reset pram. Somewhere suggested deleting extension USBOverdrive.kext but I don't see that file. Mouse works fine otherwise. Doesn't matter what browser or app I'm using (doesn't work in Word for example). 3 year old imac using mouse that came with it, running Yosemite 10.10.5.

ideas??

Many thanks!

Pat