The first place to check for such situations is in Device Manager. Go to Start > Run > devmgmt.msc. Make sure the Bluetooth receiver is plugged in. Once the device manager is opened, locate any yellow items in the list. Most likely your mouse misses a driver and needs to be installed. You can use your keyboard to navigate. Press TAB to select the list, and then the arrows to select the item. Once the item is selected, press the button between the right ALT and CTRL. Choose Update driver then Windows Update.
After installation reboot your PC and pair the mouse again. It should work. If you have a USB mouse, it's even better because it simplifies the above process.