I've had this issue for quite a while where my 275 will just spin at 100% on boot, for 5-10 seconds and then post.
I've just had a blue screen where I had to switch it off. It now does not post and the GPU fan is at 100%.
I havent changed anything and my rig has been the same for quite a while.
I'm in the process of resetting the CMOS and unplugging it for 20 minutes, and i'll try to get it to POST.. but I'm highly doubtful.
I've swapped the ram about, checked all the connections, and taken and reseated the GPU with no joy.
Any ideas might help but i'm tempted to buy a new GPU. I hope its not the Motherboard..