Phew. I'd tried unplugging from the wall, with no change. Looked up how to clear the CMOS for my mobo, shorted the pins with a screwdriver, then plugged the system back in. I think the first boot might've failed (I could be mistaken), but the next time I fired it up with monitor connected I finally got a POST screen. Froze up when I tried selecting 'continue system resume', but when I deleted the restoration data, started up beautifully. Thanks so much rolli, I don't know if I'd have thought to reset the CMOS before wasting money on another PSU.