Sometimes Mac displays will show a blank, grey, black or blue screen on or after startup.
This article offers some troubleshooting ideas to try, as remedy or further test.
If your Mac starts up to a blank screen - Apple Support
//support.apple.com/en-us/HT210905
And in the course of troubleshooting, other areas are covered too.
Contact us
If the issue continues, contact Apple Support.
If your Mac doesn't start up all the way - Apple Support
//support.apple.com/en-us/HT204156
A hardware issue may be the cause of this symptom; a diagnostic
test may be performed to see what might contribute to problems.
Use Apple Diagnostics to test your Mac - Apple Support
//support.apple.com/en-us/HT202731
You may also seek an expert to help troubleshoot, a genius at
an Apple store; or an authorized Apple service provider..
Genius Bar Reservation and Apple Support Options - Apple
//www.apple.com/retail/geniusbar/