booting up?

After reinstalling xp and doing as was advised to reinstall drivers,I now have another problem? when I try to start up computer all seems well but it isn't picking up the hard drive to install, as if there is no power going to it although there is power to fan etc.and the lead to the hard drive has power.
Just a blank screen?
where do I go from here,I can't go to safe mode because??(well) I don't know it just won't start????