won't boot from internal HD

Hello everyone,
First of all I'm sorry for my rubbish english, I'm from brazil and it's been a while since I've written something in your language. Here goes my problem:

I have an iMac with Intel Core 2 Duo 2.4GHz, 300GB, 2GB of ram, everything came with it. I didn't change a thing. This morning I left it on to download some stuff and left for school. When I arrived a couple hours ago the screen was frozen and I restarted it, but it wouldn't boot. It kept showing me a grey screen in which a darker grey folder with a question mark in it kept flashing. I went to apple support page, in another computer obviously, to see what I should do. After a long time rummaging for some answer it said to safe boot it. I tried to no result. Then it said to reset NVRAM/PRAM, which I did to no result again. The last resort was to insert the Leopard disc and reinstall Mac OS all again, which I didn't want to do because it has been a while since I've done a backup, nonetheless I did it, it was better than losing my computer. Once it booted from the CD I opened up Disk Utility to see if I could repair the disk instead of formatting it, to my surprise there was no hard disk showing up. It only showed up the Leopard disc. Then I went to System Profile to see if anything showed up there and it didn't recognize any Serial-ATA devices, it said there had been an error while trying to scan for Serial-ATA devices. That was when I decided to reinstall Leopard, I didn't realize I wouldn't be able to since it hadn't recognized any hard drive. Now I don't know what else to do. The Apple Support page is a c*** and here in Brazil there's nowhere to call to get an assistance. Hope someone out there could help me.
Thank you.

By the way, I took some pictures of the screen but didn't know how to post them, if anyone thinks it would be useful I'd be glad to put them on the post if anyone teaches me how, I'm kind of new to this posting in forum things.

Your English is fine. (Better than some native speakers.)

I hate to be the bearer of bad news, but it appears your hard drive may have failed. To make sure it's not something else though, run the Apple Hardware Test. See the following Apple KB article: Link

If it indicates that the hard drive is at fault, you'll need to replace it. I don't know how handy you are with repairing things yourself as replacing a hard drive in an iMac can be a task. The entire display must be disassembled before you can get at it. There are numerous step by step videos on the web which show how to do it. You might want to take a look at one before deciding.

I don't know where you are in Brazil, but there should be authorized Apple repair service centers there. If your machine is still under warranty, it would be best to let Apple swap out the hard drive.

Regards.