The connection between the Mac OS X and Bluetooth devices such as Magic Mouse or Bluetooth Keyboard is crashed and cannot be manually reconnected. Do the following:

1. Starts the Terminal.
2. Kill or end the Bluetooth service by: – sudo launchctl unload /System/Library/LaunchDaemons/com.apple.blued.plist
3. Then, start up the Bluetooth service again by: – sudo launched load /System/Library/LaunchDaemons/com.apple.blued.plist

Now, the Magic Mouse connection can be reconnected manually.