Try these three things, one of them tends to work.
  1. Uninstall and reinstall Citrix
  2. Upgrade to the newest version of Citrix (link at the bottom of the login page)
  3. Try it in a different browser (normally done after already upgrading Citrix)
If the steps above do not resolve the issue, please create a support request at providing the details of the issue, steps taken, and referencing this solution.