I checked iis & CM service & found ok.
But out of sudden started receiving HTTP 400 BAD request if tries to access 79xx phone properties pages.
Enter the IP address instead of the hostname.
It may be many reasons for this error.
The request contains an invalid content length header or invalid entity chunk length.
The cumulative length of the headers in the request is longer than the maximum allowed.
The header field in the request is too long.
so better you restart the IIS service.