There are two parts to this. The first is that you need to be sure that your XPath is correct.
You can probably use a tool like Firefinder (a firefox extension for Firebug) to validate the XPath.
Navigate to your page and be sure the element you need is present and then hit f12 and go to the FireFinder section. Enter in the XPath you are using and see if it can be found.
Assuming your XPath is correct then you might want to investigate using an explicit wait if for some reason you are waiting on the page element to load.
WebDriverWait wait = new WebDriverWait(_driver, 10);