A good thing to try first is a hard reset of the computer. The steps are different depending on if your battery is removable or not. See http://support.hp.com/us-en/document/c01684768 for steps and videos that cover this easy step
If your computer still cannot start despite exhausing all the steps, it may need to be serviced. Please, post in the HP Community for assistance - include as many details as possible. Make a different between "Notebook does not start" or "My Windows does not start"