Jagex wouldn't have hacked you; in this case, the site URL which you have included within your post is actually a phishing site - a site that is not the official Jagex site, but rather a look-alike site designed by the person who hijacked you attempting to trick you into entering your RuneScape account information on it. This site was then able to capture your information, relay it to the hijacker, who logged in as you and took your items.
What I would recommend doing is reading through the following support links:
The URL which you have can be forwarded to the email address '[email protected]'
If you need further assistance with securing your account and computer, its recommended to create a thread within the
Community Led Account Help forum
.