I must have been half asleep when I wrote my last post, suggesting things you can't possibly do without a visible screen.
Anyway, it sounds like your graphics output has completely failed.
Does it use a graphics card or is it onboard?
Could you borrow a graphics card from someone just to see if you can get a picture from it?
In your first post you say it sounds like it is starting up OK, does it stay on until you turn it off?
Its a long shot, but if you have more than one memory stick try starting with just one fitted and then if no change swap the sticks.