Q: Blank screen, HDD running, no response from keyboard startup combos ...

Shut down three weeks ago, attempted to restart today and getting only a faint backlit screen. Startup chime works, hard drive sounds like it's running, a USB flash drive lights properly, so the "innards" appear to be working properly. I've tried various keyboard startup combinations suggested in articles and other posts, to no avail. Once in while a crisp white Apple logo appears briefly on the screen, then the machine shuts itself down and attempts to restart, also to no avail. I have a Seagate backup drive connected but no idea how to access it. Suggestions? Thank you.