remove the batteries, wait a few mintues, reinstall, and turn back on.
Do a full restart on your MAC, and try to reconnect.
If an Apple Mouse and still having issues, contact Apple Support.
Get Support – Apple
Apple Support
Call Customer Support (800) MY–APPLE (800–692–7753)
or online Apple Support
or call AppleCare Support at 1-800-APLCARE (800-275-2273)
Outside the USA—Contact Apple for support and service by phone
See a list of Apple phone numbers around the world.
Contact Apple for support and service - Apple Support
If a 3rd Party Vendor Mouse, contact the manufacturar for Support.