My Mac opens with the sound but the screen stays black.

This has happened to me 4 times. The next day my Mac opens correctly, but it takes 36 seconds before the apple appears on screen..

What could be wrong??

Yours sincerely..

I would suggest resetting the System Management Controller and also the NVRAM/PRAM
https://support.apple.com/en-us/HT201295
https://support.apple.com/en-us/HT204063

When this happens to me, one of two things usually wakes up the screen: turn off the monitor, or Control-Escape-Option. With the first one, of course you have to turn the monitor back on, and with the second, just close the Force Quit window. But then, you didn't say what kind of Mac you have, so the first one might not be an option for you.

It's a Macbook Pro Retina 15" 2013

If you are able to run Disk Utility and check the drive to see if there are any errors. I'm not so much concerned with permissions errors as directory structure and other errors.

Flemming V. said:

It's a Macbook Pro Retina 15" 2013

Click to expand...

That model and year MacBook Pro is known to have graphics problems. Make a genius bar appointment and have Apple run diagnostics (the diagnostics are free). If it turns out to be a bad GPU, it may be covered even if the machine is out of warranty.