Shut down your Mac
Press the power button to turn it on
Immediately press and hold one of these key combinations:
Command + R to start Recovery Mode
Option + Command + R to start Internet Recovery and reinstall the latest compatible macOS
Shift + Option + Command + R to start Internet Recovery and reinstall the macOS that came with your Mac or the closest available version
Keep holding the keys until you see the Apple logo, spinning globe, or Recovery screen
Release the keys when the Recovery screen appears
Select a Recovery utility from the macOS Utilities window