Problem

Something happened and going to http://servername:5555 no longer works. Server is up and appears to be listening on that port.

Solution

Someone may have removed the redirection index html page in the Default package. Find a good copy from another IS under Default/pub and copy it in.
Or if it's urgent: go to /WmRoot instead of / (notice the capitalisation though!)

Going to the root of the admin "website" actually goes to the pub/ directory of the Default package. The admin functionality is under WmRoot.