HELP PLEASE! My mouse stopped moving the cursor

First steps are always...

1. Turn Bluetooth off/on and re-pair the device
2. Turn off the mouse and turn it back on
3. Restart the computer

You didn't say what you have tried, so that would be my first thoughts

 

If you have already tried all of that, then yes, I guess it is possible that perhaps the sensor is dirty. Try a little compressed air onto the laser on the bottom. If all else fails, an apple genius might be able to help at the store.

 

Have you tried a known-good set of batteries?

 

Sounds like there's dust on the lenses. Remove the battery cover and, preferably using a an air bower (the kind used for removing dust from cameras) blow out any dust that has collected from around and on the lenses. You may also need something like a Q-tip to thoroughly clean the lenses. Don't use compressed air - it can do more harm than good by depositing a liquid coating!