I have used horizon before and it has been perfectly fine. However, one day i tried turning it on and i get this (https://gyazo.com/02f6119d8213544e97035ceee88efbe8). No matter what I do (Uninstalling, turning off firewall, etc) nothing works. I cannot live without my horizon!
The first thing to do is to check if you have any Windows updates available, if you do, take them.
Secondly, you could try a fresh install from this link: https://www.horizonmb.com/client/v2/latest.php, just be sure to uninstall your current version first.
Try those 2 possible fixes and post back if you need more help.