Q: MacBook Guest user stuck on black screen with cursor

You could try starting up in OS X Recovery to access that version OS X Utilities and use

the Disk Utility to see about repairing disk and repair disk permissions. It may also allow

you to choose the macbook drive as startup volume; once in there, if you do not have an

automatic login (you see all users, login to any with their own password) this would be OK.

• OS X: About OS X Recovery - Apple Support

If the main Admin user is set for the automatic login, then it would be first choice on restart

especially after repair disk permissions from the booted OS X Recovery's utility. As this

'recovery' runs from a different partition on the hard drive than the regular OS X system,

the system is unmounted and can be addressed by this version of OS X Utilities. Be careful.

• OS X -Tips Using the Recovery HD:

http://pondini.org/OSX/RecoveryHD.html

Which MacBook model version are you using? If the unit is older than the 12-inch with Retina

display, then other ideas may apply. However given this thread is about the recent 2015 model

that should be a given; sometimes topics are posted to incorrect sections in ASC discussions.

You may be able to get a rise out of it by resetting NVRAM, perhaps not. I'd try recovery first.

How to Reset NVRAM on your Mac - Apple Support

You may be able to try and use the startup keys to make SMC work, but that may be a stretch

and it may not make a difference in the situation. Some of its intended purpose is power mgmt.

This does different things and may be a good reference for a later date:

Resetting the System Management Controller (SMC) on your Mac - Apple Support

These kinds of things are easier with the product in front of the person suggesting things; in

part due to the time it takes to write a suggestion, test the link to methods in Support, and

then not know if anything else had been attempted before or after that one inquiry was made.

You may also consider an official Apple retail Store with Genius bar, set an appointment for them

to see you about it; take the power supply along in case you want them to test it, too.

https://www.apple.com/retail/geniusbar/

Good luck in this matter!

PS: since the above timed-out while I was called away for

an offline adventure involving instant coffee & sugar, here

is another idea that may help on restart, to gain access...

Another option to try, may be Startup Manager on boot

How to choose a startup disk on your Mac - Apple Support

Use these steps to choose a startup disk with Startup Manager:

1. Turn on or restart your Mac.
2. Immediately press and hold the Option key. After a few seconds, the Startup Manager appears.
3. If you don't see the volume you want to use, wait a few moments for Startup Manager to finish scanning connected drives.
4. Use your mouse or trackpad, or left and right arrow keys to select the volume you want to use.
5. Double-click or press the Return key to start up your Mac from the volume you selected.

Startup key combinations for Mac - Apple Support

Good luck & happy computing!