Does this have Vista on it? If so, youll need to download the newest wifi driver and update it. I have seen this issue on several Vista based laptops. Also, follows these possible solutions.
1. Some HP's have a wifi switch on the side or near the front. Flip that on. These can be turned on accident.
2. See if there is a function button to turn on your wifi. For example, FN and F9 to toggle wifi on and off.