Mac OSX not starting don't know what to do.

Okay here is the problem. My husbands computer has not really been used much, mainly just to surf the web. Well now for some reason everytime we try to turn it on all we get is the grey screen with the loading circle, all it does is keep spinning, have read the manual and been on the apple website but none of their suggestions are working. I don't know what to do now. Please help.

Do you have the installation CDs / DVDs ?

Boot with Apple Hardware Test disc if you have one (keep 'c' pressed when booting to boot from CD). Run the tests to check if the hardware works.

Then boot with OSX DVD and run Applications - Utilities - Disk Utility, verify disk permissions.