iMac won't boot an OS

Ive got an early 2008 iMac, and when i went to turn it on today, all i was met with was a blank grey screen. There was no apple logo, no spinning loading circle or no folder icon, just blank. Its currently running OSx mavericks, but it came with snow leopard. Ive tried booting from the snow leopard disk by holding c at start up. Ive also tried booting in safe mode by holding shift, and single user mode by holding cmd + s. I can also hear a small beeping noise that beeps once every 2 seconds or so if thats any help? Can anyone suggest what i should do next? idealy i dont want to have to take it into a shop, as the nearest apple store is about an hour away, and i can see it getting very costly for me to take it to a store.