mijntje024 You cannot build a community on our demo sites, as luceos stated, the data will not persist, and it is not intended to be used for personal communities. If you have given anyone that link and promoted it as your site, then that was an error. The demo sites are intended to demonstrate the features of Flarum for you before you create your own site.
To get your own version of Flarum, you will need a web host capable of supporting Flarum and then follow the installation instructions on that link. You can also utilize a project by a Flarum community member, FreeFlarum.com and get setup quickly with the managed version of Flarum there (which does not give you configuration/full control, but does allow you to build your own community with the tools it provides).