So I really have no idea where to start, I can usually fix things myself but I can't even get to somewhere useful.
I have made no hardware or firmware or any changes since last successful boot. Mac OS X boots up fine from my second HDD (my laptop has PC architecture, it's not Apple). Removing this second (OS X) HDD and then attempting boot makes no difference.
What gives? If I can't boot from a Windows USB then...it must be hardware related? But Windows was working fine with exactly the same hardware for ages. Plus it's all still recognised in Mac OS X, so nothing has suddenly broken. My 4x4GB RAM SODIMMs are still fine, so is the CPU and GPU, sound card, wireless card...I really don't know what I'm looking for.
I might have to start removing things and swapping them about to see what happens.
edit: feel free to move to Crashes and Debugging