Hello, I have an iMac and it crashed a couple of weeks ago. I was able to get it going and save most of photos. It was working just fine and last night I was copying my photos to a disc and suddenly the color wheel came on, the computer froze and when I try to turn it on using all the different ways on the Apple book, nothing happens. The screen is blank and there is a question mark - Ithink it's the hard disk. I've exhausted all the ways of turning it on but nothing happens, just a blank white screen.
More aboutapple computer crashed
Unfortunately what you are describing sounds like the hard disk. Are you comfortable opening it? If so, on the 20 inch imacs here is how to get it apart.