replaced hard drive for a gateway

ok..i have been helping on my computer teacher during my study hall in school and we both have stumbled upon a problem that has us baffled. I have been working on a Gateway 500 series that supposedly has a bad hard drive. so i replaced it with a new hard drive sent from Gateway. as far as i can tell and i followed the instructions on how to replace it off of the gateway website, i have put the drive in right. but once i put in all the cords and then attempt to turn it on, all i get is an orange light. nothing seems to work. the fan doesn't start up or anything. i'm just a baffled person hoping to find the answers...