Oh dear, that's not good
Your PC must, at least sometimes, know that your hard disk is there and that it should use it because you said that it sometimes does display the Windows ME logo screen. That means it's found the hard disk, read a couple of files from it and decided that it's supposed to try loading Windows.
However, you might find that the IDE cable that goes from the motherboard to the hard disk is loose. It might have just been a coincidence that you had a power cut - 'red herrings' are common when trying to diagnose PC problems!
Unplug the PC from the mains, remove the system case and find the flat grey cable that goes into the back of your hard disk. Check that it is securely fitted at both ends (one end in the hard disk, one end onto the motherboard).
Other than that, it sounds pretty terminal
Do you know what hard disk you've got fitted (make and model number)? There may be a manufacturer's diagnostics program that you could get. If you don't know what hard disk you've got, look at the label on the top of it while you've got the system case off (you might need to unscrew the hard disk and pull it out to see the label).