I have an iMac from 2011 and I have installed Yosemite as soon as it was released, without any issues before this one.
Here is what happened: it has suddenly shut itself off, after turning it on, it goes into the loading screen, loads to about half of the loading bar and restarts itself, this happens in a loop.
I have tried internet recovery. After choosing the network, connecting, and going through the loading screen, the screen becomes grey. Same thing when I try recovery mode or when I boot from external usb device. I am able to enter the verbose mode and I was able to do system check, which did not find any issues.
Not sure what to do and whether this is a hardware or a software problem.
Best Answer
These symptoms imply an issue with your GPU. There is actually an ongoing class-action lawsuit against Apple regarding the AMD GPUs in the 2011 Macs (primarily MacBook Pros). I have seen the issue before in person; my sister's MacBook Pro just encountered it. The symptoms were just as you described: boot chime, Apple logo, spinning loader, grey screen, nothing else. Apple had recently started using lead-free solder for internal components in order to adhere to EU environmental regulations, but the solder is more prone to stress and cracking from rapid heat fluctuation, which can cause the GPU to slowly separate from the logic board. If this is in fact the case, you have 3 options:
You can read more about the lawsuit here: http://time.com/3545723/apple-macbook-lawsuit/