@Dietmar_Mauer1, welcome to the Community. What devices are you using to try to connect to your Nucleus, and are they connecting via Ethernet cable or WiFi? Also, what version of Roon are you using on your Nucleus, and what version are you using on your Remotes?
Ben here with the support team, you have my apologies for the delay in getting to your thread!
Following up, I’m not seeing your Nuclues within your account information. If possible, could you attempt to bring it back online? And with that, if you attach a monitor to the Nucleus via HDMI, does anything show up on the screen?