trouble with my g5

I have a G5 which operates on MacOSX which i bought back in november, and as of yesterday it was working fine. however this afternoon i tried to start the computer up and after making the typical sound that every mac does, the only thing that would appear on the screen is an icon (the finder icon) and a question mark in it too. if i leave the computer long enough, then the fans start going faster and faster and making more noise, but nothing actually happens. Do you by any chance know why this is happening? Thank you very much for taking the time to read this and answer me.