Flashing folder w/ question mark :(

I have an iMac 06 20in and earlier this morning i was on the internet and it just froze, so i held in the power button to turn it off..when i turned it back on, i heard the startup sound but it was just a blank grey screen

After about 5-10 mins, it shows a flashing black folder with a question mark where the apple symbol usually is

I have tried the unplugging for 30 secs thing, holding option, command, p, and r, and even holding in c (all were recommended on apple/support) and nothing worked

Like i said this is the first time this has happened after lots of use over 4 years of owning it, and its pretty frustrating...

Any suggestions will be greatly, GREATLY appreciated

Thanks in advance

Welcome to Mac-Forums
The flashing folder with a question mark means it's unable to find the operating system. Boot up the OSX DVD you have with it and use disk utility to repair the disk and then repair permissions, should solve the problem, if not then get back to us and we will help you further.

- Simon