Couple things to check,
If you have multiple Macs, make sure it's not paired and connecting to the other one.
Make sure you inserted 2 good batteries both with the + end facing up.
Look in System Preferences > Bluetooth > see if the mouse is still paired, if not, turn the mouse off and then on and click the "+" sign below the list of devices to pair it. You might need to click the mouse for it to show up.