Reboot
How do I reboot or re install my computer
iMac, iOS 10
How do I reboot or re install my computer
iMac, iOS 10
Apple menu -> Restart
Reinstalling macOS depends on which version you are running, and how it was installed. If you are running OS X Lion 10.7 or newer, then
Boot holding Command-R
If your Mac is newer than about late 2011, you may also be able to do Network Recovery Mode
Boot holding Command-Option-R
If the operating system is the original operating system that shipped from the factory with your Mac, you should be able to re-install it.
If you downloaded your version of macOS from the App Store, then it should still be available using the same Apple ID you used to download it the first time.
If you want the latest and greatest, then you can always just download and install that copy.
If you previously downloaded the macOS version you want to re-install, then go to the App Store -> Purchases, and download a new copy.
Reboot