Q: Mouse quit working

Ensure that the batteries are installed correctly. Apple Wireless Keyboard, Mouse, and Trackpad: How to install batteries - Apple Support

Have you tested the "new" batteries?

How old is the mouse?  I had a 6 year old wireless mouse die on me last year.  They do wear out.   You can always take it into an Apple Genius Bar and have their designated veterinarian look at it.  That's what I did.  Then I had a nice burial ceremony afterward.