Hi David, I'm not a computer wiz like a lot of the folks in these parts, but I think more information would be needed. Did you make any changes lately? When did the problem arise? What is the make and model?
I have one thing to try, if you have a linux live cd (you can just download and burn one), you can boot from that. If that works as expected, than the problem is likely software. If it doesn't work, it could be a couple of things. The first is that your computer isn't 'linux compatible', that happens. The second would be the worse one that maybe you have a hardware issue.
That's really the extent of my computer repair prowess.... hopefully someone with some more experience can help ya out.