If a user cannot connect to a Citrix server via RDP or ICA (assuming your server is setup for RDP / ICA), your ICA / RDP listener might be down. To fix the RDP / ICA listener so users can connect to the Citrix server:
Connect to the affected server using Terminal Services Manager from another server
Expand the Citrix server that users cannot connect to
The RDP-Tcp or ICA-tcp session will not exist, but at the bottom of the list there will be a session that is down as the example below:
(the port number the RDP session normally listens on is 65537)
Click once on the down session
Right click on the RDP session and select Restart
The user should now be able to connect to the Citrix server over RDP / ICA. If the user still cannot connect to the Citrix server, you may have a configuration connection issue or may need to restart the ICA service (:!:Warning, kicks off all user sessions).