Hey there.
What happened? Did you just change the old hard drive with a new one without installing an OS?! If this is the case it's normal for you not to be able to boot to Windows. You can create install media by downloading Windows form Microsoft's website. Here's an article which has all links included, since you haven't specified which version you need: http://www.howtogeek.com/186775/how-to-download-windows-7-8-and-8.1-installation-media-legally/. Note that you should install the exact same version of Windows, your laptop came with. After that check your OEM Windows key, which is usually located on a sticker on the bottom of the laptop. If you have no key there, I'd suggest that you contact the laptop manufacturer's customer support and ask for further assistance.
Hope that helps.
Boogieman_WD