This is most commonly caused by, either the firewall blocking any kind of traffic towards the VPN server IP address or the FortiClient application itself by the firewall on the host or on the network, or either by routing errors towards the IP address of the VPN server.
The problem can usually be solved by adjusting the host or network firewall rules on the client side.
Sometimes in rare cases I have found the problem is caused by error on the FortiGate device, in this case no one is able to connect to the VPN neither using SSL VPN or IPsec but the internal networks can go to all local networks and the external internet connection. In that case a simple reboot of the device solves the problem.