This isn't really a Photoshop task. But I can help you out if you still have the URL.
Right click your web page and then click "view source" or your browser's equivalent. Ctrl+F and search for "css". Read through the CSS section until you find the font or font family. If there are multiple, try them all until you get the right one.
Or you can go to the web page, zoom in as much as possible, take a screenshot, and run it through the FontSquirrel Matcherator online.