My 2007 iMac won't turn on

Hi,

I've been reading the forums trying to troubleshoot but no luck. I bought this iMac new in late 2007. Just a couple of months ago it shut down and wouldn't turn back on. I was looking at the back panel and noticed a vent behind the stand that was clogged with dust (I guess I had totally forgotten that vent was there), so I think the machine must have overheated, although I've read on a few Mac forums that the iMac shuts itself off before overheating to avoid permanent damage, is that correct? But anyway as I said it won't turn back on so I am worried that something may have been damaged after all. I have tried doing an SMC reset as recommended on the Apple website, to no avail. Is there anything else I should try before I bring it in for repairs?

Thanks in advance.