It's unlikely there's something wrong with your mouse if it was working fine until the last set of batteries went kaput. Try this:
Remove the batteries. Be sure the mouse is OFF.
Go to System Preferences->Bluetooth and remove your mouse from the paired list. Then turn your Mac's Bluetooth OFF. Now restart your Mac.
Put the batteries into the mouse.
Go to System Preferences->Bluetooth and turn your Mac's Bluetooth ON.
Turn the mouse ON.
Select SET UP BLUETOOTH DEVICE from the preference pane or from the Bluetooth menu bar item.
Another frequent problem is that the battery cover isn't fully seated. Make sure you press the cover closed far enough that the tab comes through the cover and "clicks". If not fully closed the batteries can lose contact.
i'll try, but i can't see how that could be the problem - the mouse won't even turn on. the little green light does nothing, so how could it be a pairing issue? i've also tried repeatedly to reseat the battery door, with no effect. it almost feels like the switch went bad. it just feels funny, like it's not clicking when i slide it.