Turn your Bluetooth keyboard and/or mouse off for 30 seconds, then back on.
If the keyboard or mouse doesn't respond, or if an alert message appears again:
- Connect a USB keyboard or mouse.
- Open Bluetooth preferences (choose System Preferences from the Apple menu, then click Bluetooth).
- Verify that Bluetooth is turned on. (You can also do this with the Bluetooth menu bar item, if enabled).
- Pair the keyboard or mouse by setting it up as a new device.