update: I swapped a hdd from another pc into this one and it worked, suggesting a hdd problem. However, upon putting other components back into the pc (graphics card and the other ram stick), it stopped working, and even though I've removed them it has returned to giving no video output. Back to square one. I can probably rule out a broken cpu or mobo as it managed to boot successfully a couple times. Any thoughts?
Hands-on: Samsung Galaxy S8 review