I had my Macbook repaired and when it came back to me, it's running on Windows already. How do I revert back to my Mac OS?
Has MacOS been deleted or are you running in Bootcamp. Try restarting with the option (alt) key held down to see if you are presented with a choice of boot drives, and select the MacOS Drive.
Depending on which version of MacOS you had installed you may also be able to access the recovery drive
If MacOS has actually been deleted then take it back to the repairer and ask them to reinstall it...