What make and model is your mouse? Have you installed the correct drivers
for that device? Using a separate Bluetooth USB receiver or built-in Bluetooth? Are other Bluetooth devices working?
Most common reason for a Bluetooth device to suddenly stop working is that the computer can no longer receive a Bluetooth radio signal. This can be caused by empty batteries on device, unpluggeg or disabled Bluetooth receiver, or another wireless device using a frequency too close, disturbing the signal. So, check that USB dongle is plugged in if using it, or that WiFi / Bluetooth is turned on if using built-in receiver (in many laptops, WiFi and Bluetooth are connected together so that if you turn for instance WiFi off, also Bluetooth stops working).