If your Mac has a blue screen or other startup problems
Login items are apps that open automatically when you start up or log in to your Mac. Occasionally, a login item may cause a blank blue screen at startup or other problems.
Note: To print these instructions, press Command-P.
Test your login items
On your Mac, choose Apple menu > System Preferences, then click Users & Groups.
Click your account name below Current User, then click Login Items at the top of the window.
Make a list of the login items—you’ll need to remember them later.
Select all of the login items, then click the Remove button .
Choose Apple menu > Restart.
If this solves the problem, open Users & Groups preferences again, add the login items one at a time, and restart your Mac after adding each one.
When the problem occurs again, follow the steps above to remove only the last login item you added.