Things to check,
Windows 7
Press FN+F5 on your keyboard and make sure the Bluetooth status is ON.
Windows 8
Pull up the Charms bar
Select the Settings charm, and then click Change PC settings.
Select Wireless.
You can now change the Bluetooth setting to On or Off.
Check your mouse about paring mode or connect button underneath, (Some mouse has a trigger to pair)
Make sure that the Bluetooth services are started
Press Windows button +R
Type services.msc
Look for any Bluetooth related services and make sure they are on automatic and running.(system reboot needed after changes)
You can follow the guide on these articles
Windows 8
Windows 7
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