Most likely, you have IVPN Firewall (Kill-switch) enabled. It was designed to ensure that all traffic is routed strictly through the VPN tunnel to prevent any possible leaks or your real IP address getting exposed. Given the latter, it will also block the Internet connection if you disconnect from IVPN network.
To regain access to the Internet, you must either disable the IVPN Firewall or reconnect to any of the IVPN servers. You may also control its behaviour within the app's Settings area - IVPN Firewall tab.
Feel free to read more about how our Firewall works here