It's difficult, if not impossible, to tell WHY it doesn't work anymore. It suffice to know THAT it doesn't work anymore to replace it. Mice that go south aren't repaired but thrown away.
If it's still under warranty, you get the new one free, most likely.
It came with the new computer and worked fine. All of a sudden, it no longer left clicks. I have tried putting the toggle into another usb port and also rebooting my computer.