iMac won't load operating system

I'm using one of the iMacs that came out before the intel versions with the latest operating system version, but when I try to start up my computer, I get to the screen that says Mac OS X that usually loads, but nothing happens. It just sits there and won't respond to anything. How do I fix this?

And i think I'm using 10.4 actually, not 10.5

Throw in the system disc that came with your computer. Hold the "C" key down as you boot up your machine and I'll book up from the cd and from there you can go into the menu items and do a hard drive repair on your machine. That should fix most things. And if that doesn't fix the problem it might be worth taking your machine into a mac store or showing it to some mac expert you know.

If what the8thark's suggestion with the install disk doesn't work, there are a few things you could try before taking it in repairs, one of them being:

Resetting your Mac's PRAM and NVRAM

Can you recall when your iMac started to exhibit these symptoms? Was it right after a software installation? Did you modify any settings in your system or did you add any new peripherals? Do you let your iMac on at night so it can run its built-in maintenance tasks or, if not, do you run them manually?

Just for safe measure, can you also insert your install DVD with Apple's Hardware Test on it? Restart holding down the C key and run the test in extended mode, this will make sure all hardware components on your Mac are ok and working properly.