Hi
You don't give specific details or tell us what troubleshooting steps you've tried but guessing a little from what you've written it sounds like you're missing some configuration steps.
If the ssl vpn tunnel is connecting, but you lose internet connectivity, my first thought is that you're not using split-tunneling and don't have the correct firewall policies for that, it also makes me think you're missing the correct firewall policies and\or routing for access to your internal network. Another possibility is subnet overlap.
Try reviewing the configuration guide and see if you've missed something.
[link]http://docs.fortinet.com/d/fortigate-ssl-vpn-3[/link]