iMac shows GRAY, BLUE or BLACK screen for more than 15 minutes.

If this happens, there has been a severe issue on the MAC's hard drive.

Only one thing to try in this case:

1. Unplug the power cord from the back of the iMac.
2. Press and hold the powerbutton for more than 5 seconds. This will drain the motherboard's capacitors of all energy, resetting all components.
3. Reconnect the power plug, take a deep breath and push the power button normally to turn on the iMac.

If this solved it, and after max. of 15 minutes the MAC desktop comes up, good.

Why about 15 minutes? This is the maximum time it should take the iMac to check the hard drive for errors. It normally does this during startup showing a gray screen with a rotating gear.

If not, sadly, we need the MAC back to further troubleshoot the cause. Please give the tech depaartment a call then.

Please read "Shut down an iMac properly" to avoid this issue happening with your replacement Mac.