The site is up and running: https://demos.telerik.com/blazor-ui/grid/overview
We did update it today, to the newly released 2.1.0 version of Telerik UI for Blazor, which added support for .NET Core 3.0 RTM. During the upload procedure there might have been a slight downtime, but no more than a few minutes. Perhaps it was a bad timing issue?
We would be happy to hear if you are still unable to open the site, or if you have any further issues with it.
UI for Blazor