You should have gotten a DVD for server reinstallation with the purchase of your Mediaserver. This must be executed from a client operating system. Which client OS are supported here, may depend from the model, therefore check the manual which you have gotten
with the box or find the appropriate documentation on the HP website.
(In one case years ago for the very first model I had to use a virtual PC with Windows XP to trigger a reinstallation.)
Best greetings from Germany
Olaf