First try restarting the computer that uses this mouse. If it fails to start working:
un-pair the connection... restart machine again... then repair the mouse with the computer. You should be good to go. If not, then you either need to check to see if you need to do any updates...
Worst case scenario, the hardware in the mouse went bad. In which case you can either do a tear down and attempt repair... or search macsales.com or amazon for a replacement. With Black Friday around the corner deals are all over the place. I believe I saw one as cheap as $30.
I'm not sure that it would pertain to this situation...
If you want to try a SMC reset prior to a teardown or replacement... make sure that you have a backup of your drive and the data on it. Then do a search for " SMC reset" and the model of your Mac computer. Click on the link that leads you to the apple forums. There you will see a full example of how to do a SMC reset.
Sometimes things like SMC resets and even PRAM resets tend to knock out any weird "cobwebs" that tend to bother Macs.
Happy computing and good luck.