I I can't launch the jupter notebook from anaconda navigator

I can’t launch the Jupter notebook from Anaconda Navigator, although I was able to do so yesterday. I got the following message:

 Sorry, this is not working properly!

Our server encountered an unexpected condition that prevented it from fulfilling your request.

Internal Server Error

Failed to connect to Hub API at ''. Is the Hub accessible at this URL (from host: green-nbjupyterhub12)? Make sure to set c.JupyterHub.hub_ip to an IP accessible to single-user servers if the servers are not on the same host as the Hub.

I tried again and was able to launch Jupyter Notebook from the Anaconda navigator, but the other problem occurred: the kernel selection dialogue box did not allow me to select any kernel.

Now the Jupyter Notebook can be started, the kernel can be selected, and all of its functionality has been restored.