If you are interrupted, even briefly, with the Horizon import window open, your screen may freeze and you will not be able to do anything in Horizon.
Easiest solution is to shut the computer down for 5 minutes and let everything clear with the citrix login. Faster option is to double click the citrix icon in the lower right corner, click on the down arrow by the arrowhead# (remember that number) and click on Log Off, then go into the task manager and close all listings for Citrix (including several that may be listed under "Background processes), wait a minute or two and then log back into Horizon you will probably need to enter the user name "dynixasp\arrowhead##" (the number you remembered from earlier) and the password.