Brand new 5k iMac black screen

You have a 14-day window to return it, no questions asked. But, before you do, ensure that you boot into Internet recovery mode and securely erase the HD. Additionally, you have at least 90-days of free phone/Internet support (3 years if you bought the AppleCare Protection Plan, a must, IMO, for desktop machines). Contact Apple's Support and let them deal with the problem. It might just entail wiping the HD, reinstalling the OS, and wasting another nine hours to migrate.

27" i7 iMac (Mid 2011) refurb, OS X Yo (10.10.3), Mavs, ML & SL, G4 450 MP w/10.5 & 9.2.2

I had same problem.

try unplug all USB cable, then boot.

I have a similar, if not the same, problem. Brand new i7 5k iMac (at least, brand new to me - bought from refurbished store). Switched it on and it booted fine to Yosemite 10.10.2. It immediately told me to install the latest updates - i.e. including the Yosemite 10.10.3 combo update. I clicked install - and 20 minutes later the system reboots, I get the chime and a black screen. No Apple logo, no grey screen - nothing. However, when I connect my Dell monitor via a display port - I get the login screen (or the left desktop of it) on the Dell monitor. I can log in and used the iMac, but only through the Dell monitor. When I check the System Information, it tells me that I have two displays - and the Built-in Display is now allegedly an 18" display at 2560 x 2880. But on System Report the Graphics/Displays section shows that I have a Retina LCD at 5120 x 2880 (see attached).

I think that the 10.10.3 update has somehow fried the firmware in the AMD Radeon R9 or in relation to the display. I think it's going to have to go back to Apple.

Any ideas otherwise?

The Same thing happened to me . I just purchased a Brand new late 2015 imac 5k retina 1tb Fusion and was so excited. Well excitement turned to pure frustatuon. I would like to add this is an 2 exchange in 2 days. I came home placed imac on desk and booted up fine and then allowed the apple update and when the imac restarted it went to a black screen. I pulled powered and everything else. Spent nearly 2 hours with. Apple care with no resolution. Finally gave up and returned it the next morning, very disappointed. I can't explain how bad I felt no one could help me. Not to mention 2 in a row! I'm don't know what to do now. I couldn't bare a 3rd try at another one. 6 hours in the car driving to the store to replace these brand new imacs. Just speechless...

The Same thing happened to me . I just purchased a Brand new late 2015 imac 5k retina 1tb Fusion and was so excited. Well excitement turned to pure frustatuon. I would like to add this is an 2 exchange in 2 days. I came home placed imac on desk and booted up fine and then allowed the apple update and when the imac restarted it went to a black screen. I pulled powered and everything else. Spent nearly 2 hours with. Apple care with no resolution. Finally gave up and returned it the next morning, very disappointed. I can't explain how bad I felt no one could help me. Not to mention 2 in a row! I'm don't know what to do now. I couldn't bare a 3rd try at another one. 6 hours in the car driving to the store to replace these brand new imacs. Just speechless...

My computer was fine for 3 months until I clicked yes on the update. My friend told me not to update it after he had major problems with his phone and ipad updates. I clicked yes without thinking. Walked away from the computer and came back to a dark screen. It's going to the Apple store tomorrow. I think the video card got fried. I get the chime but have no display. I can see a dark purplish light at an angle but from straight on it looks black. Just glad it's under warranty. What a disappointment. Apple makes a nice computer and has excellent service. They really need to do better with these updates. I hope I don't lose my data. I have a couple of photo shoots on it that I didn't get to update yet. They can not be recreated either.

Do note that if you have AppleCare's protection plan and you're within 50 miles (80 KM) of that Apple store, you might be eligible for an onsite repair since yours is a desktop machine (depends on where you're located). BTW, the AppleCare Protection Plan's the best warranty policy available for desktop machines. Get it if you don't have it before contacting Apple again.

why hasn't apple responded? what kind of scam is going on here!!! its time to step up apple.