If your MAC deals with booting issues, and you look at a black or grey screen instead of your desktop, then try booting in the safe mode. In Safe Mode, MacOS will boot with the bare minimum of software and drivers required and will run a check of your start-up disk and repair any directory issues that might be the cause of your start-up ills.
To start up in safe mode, start your Mac and then press and hold the Shift key. The Apple logo will appear and then the login screen. You can release the Shift key when the Apple logo disappears and the login screen appears. It would take few minutes. To exit the safe mode, start your MAC the usual, without holding any keys.