Black screen After changing Nvram preferences

Hi there, the ac was plugged in. I followed the NVRAM reset process. Make sure the mac is turned off (no light on the back white apple logo). If it turned on, press the power button until it off. Hold the 4 buttons (cmd-opt-R-P) and the power button. After a while (few sec but make sure the mac is turned on. The screen will goes into dark-purple...) release the power button while continuing pressing the other 4 buttons. Don't let it go until a chime is heard. (It took me more than 5 minutes holding those buttons).

Hi there, Ekerrbrown.

I see that the display of your MacBook Pro won't turn on after allowing Option ROM firmware to load automatically using a terminal command. Now, the display of your MacBook Pro won't turn on, and I'm glad to help.

How to reset the System Management Controller (SMC) on your Mac -- Depending on which model of Mac you're using, reset the System Management Controller following the directions for your Mac. See if your display turns on again.

Let me know how that goes, and take care!

Ekerrbrown wrote:

 sudo nvram enable-legacy-orom-behavior=1

hmmm...

The external drive would need to be "bootable" to begin with—if it does not show in the Startup Manager—then I suspect it is not bootable.

>System Preferences>Startup Disk

You do not say what exact Mac /model /year /size /retina /touchbar is this 

or

what exact macOS you are currently running? 

Thanks for trying an SMC reset and resetting NVRAM that HWTech provided, Ekerrbrown. It's appreciated. Since the display still won't turn on, it's time to contact Apple Support.

Apple - Support - Product Selection — After selecting this link, choose Mac notebooks and type "Display" in the Search topics field. Select "Display not working as expected" from the list provided, then choose how you'd like to start the conversation.

Cheers!

If the NVRAM change is the actual cause of the problem and the resets did not work, then I'm not aware of anything you can do but have the Logic Board replaced. I've had a similar problem of corrupting the NVRAM of a couple of original Intel MacBooks while trying to install a Linux bootloader (which should never be able to brick a computer).

There is a slim possibility the laptop has a failure unrelated to the NVRAM change and the timing of the failure is just coincidental.

FYI, in the future consider trying the rEFInd boot manager which is a nice third party bootloader with a graphical boot menu which can be used on a USB stick or installed directly onto the system drive.

I'm having the exact same issue. Tried to fix a sleep/wake issue with a tutorial from youtube and now it's just a light and no bootsound. Did you get any further in this process? Or is it a lost cause?

I did the same command and had the same symptom. I overcome it by holding the NVRAM rest buttons (cmd-opt-R-P) for ALMOST 5 minutes and suddenly heard the chime (I press few time the on button to make sure the mac started and continued to press the other 4 buttons). Hope this help.

Could you please describe what exactly did u do to boot?I have the same problem, but was holding RPAM reset keys for a few minutes - nothing happened. :(

Did u continue to hold keys when folder with question mark appears? Did u connect AC power or did it on battery?

In addition to the SMC reset, perform a PRAM reset (hold the PRAM reset for a full minute so it activates several times).