Thank you for your post.
With the information provided, it is difficult to determine what may have caused Admin Console to not access FileMaker Server.
For clarification, did you "restart" FileMaker Server? Or, did you "restart" the actual server machine?
Please send in the FileMaker Server Event log as well as a system log for the time period when this issue occurred. Check your Inbox at the top of this page for instructions where to send the files.
This is disturbingly similar to an issue with FMS 10 that was never resolved...