if you are running ESXi 5 / 5.5 [ I've experience only these two], you probably experience error as below image, with error message -
" vSphere Client could not connect to "XX.XX.XX.XX". An unknown connection error occurred. (The client could not send a complete request to the server. (The underlying connection was closed: An unexpected error occurred on a send.)) "
All the search with error log to google, brings result for vcenter error for IIS.
However, my server(s) is a standalone machine, where only ESXi 5.5 or 5.0 installed.
Little detail information about this error and solution could be found here.
What I basically do most of the time while I got such problem is,
after logged-in as SSH. If you've SSH service closed, you better ask you DC for KVM.