It is possible to troubleshoot these issues while in Safe Mode. Safe Mode performs a start up using only essential system resources. If you find you ever need to boot your Mac in Safe Mode, follow these instructions.
1. Shutting Down- If you are able to perform a normal shutdown, do so. If you are not able to shut down normally, press and hold the power button. Hold the Shift key if your keyboard is wired to the Mac and proceed to Step 2. If the keyboard is wireless, shut down and proceed to Step 2.
2. Starting up again- If your keyboard is wired, continue holding the Shift key and start the Mac. When you see the desktop or login screen, release the Shift key. If your keyboard is wireless (Bluetooth), proceed to step 3.
3. Starting up- When you start your Mac, press and hold the Shift key after you hear the startup tone. Release when the desktop or login screen becomes visible.