G5 won’t boot from HD

My Power Mac G5 running “Tiger” has suddenly decided it doesn‘t want to boot from the hard drive. I normally leave it on all the time, and when I tried to use it today it wouldn’t respond to the mouse or keyboard; my Studio Display was blackish (not solid black as when it’s powered off). Thinking it had just lost touch with the USB or something, I restarted the system with the power button. It briefly displayed the light-grey screen with the Apple logo, then went black, showing an irregularly blinking solid-block cursor at the top left.

I rebooted from the system DVD (no problem) and ran Disk Utility, which found and repaired some minor problems. I then used Startup Disk to choose my TechTool “eDrive” partition, from which I booted (no problem), logged in and ran the “Intermediate“ tests. All the hardware passed; again some volume-structure problems were found and repaired—except that it said Calculator had a file-structure problem and should be deleted. No big deal, I thought, and tried rebooting again. Just as before, after a brief appearance of the Apple logo I get the black screen with blinking cursor. Any suggestions of what to try next?