Yosemite install won't boot into any mode

Really sorry if this has been covered before but I spent a good time searching and couldn't find any existing posts with symptoms to the same extent.

I've got a 2008 iMac 27" and have the latest version of Yosemite installed (10.10.1 I guess). I upgraded to Yosemite probably 2-3 weeks ago and everything has been fine. Then yesterday when I woke it up from sleep it effectively froze - no mouse or keyboard input. So I did a reset by holding down the power button, and when I turned it back on it went to the apple logo boot screen with a progress bar that gets to just below 50% and stuck there. I've restarted quite a few times and left it running up to six hours and still it doesn't boot.

Now this is similar to the issue here - OSX Yosemite won't boot after install on mid 2010 iMac - except I am unable to boot into any mode, and the only startup keys accepted are the P-RAM reset. It won't boot into recovery mode (command+R), safe mode (Shift), startup drive select mode (Option), diagnostics mode (D)... nothing 😟

I've booked into a Genius Bar appointment when the Apple stores reopen after Christmas, but I'm hoping that is a last resort and to be honest I'm unsure what they can do for me except to say it's a lost cause.

Are there any suggestions on what could be happening and why it's ignoring the startup modes? I would have suggested the keyboard not being recognised except the P-RAM command works perfectly, so it's obviously seeing it. I'm a little scared that because it won't let me into a boot device menu (which must operate independent of any HDD) then there's an issue ahead of that in the boot sequence, so possibly the motherboard is shot... 😟

Sorry if I seem a little desperate. I had a HDD crash two years ago to the day (great Christmas present Apple) and it kind of puts a dampener on things.

Happy holidays and I'm grateful for any help that anyone can offer 🙂