Imac will not start

[bafish2]

I have an older imac which will not start.
I have tried to start with cd, but that did not work.
The kids have loaded so many mp3 files that i believe the hard drive has become just plain full. When you try to start you get a happy mac icon but
nothing happens it just sits there smiling. It is almost like it's saying I know something that you don't......any help?

[Alex]

What happens when you try to start off a CD? Make sure you're holding down the C key.