Remotely upgrading to El Capitan broke Screen Sharing
The three ways to connect are Apple Remote Desktop, ssh and remote Apple Events. If you have one of those enabled, you can kickstart screen sharing with a command line tool:

   sudo /System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Resources/kickstart -activate -configure -access -on -restart -agent -privs -all


See these for more details:


Restart Screen Sharing Service on Mac mini Server
Remotely turn on screen sharing
activating screen sharing remotely


If not, you will have to have the machine shipped to a human or a human shipped to the machine to take local corrective action and diagnose the failure.