I assume you mean you have a Magix account and can see listed there that you own MusicMaker. Is that correct? if so, that listing will also include a link to allow you to download the installer, assuming your version is reasonably up to date.
@Anders-Svensson2131 Just download the newest version from Magix.com and install it. When you start it for the first time just enter your mail address and password of your user account. Then it should work as normal as before.