1. SSL issues; Security issues with IE; Citrix Client not connecting

CAUSE

If it is Single User

1. Citrix Client not installed
2. IE not configured properly
3. User does not have access to Citrix Site

If it is multiple users then

1. SSL certificate expiry or Mis-configuration
2. Root Certificate Validity
3. SSL and Root Certificate Binding
4. Firewall port issue (https:443 port)
5. Secure gateway certificate issuing issue

you can Ask if you stuck somewhere to me by adding comments