There are multiple ways to fix this issue. Read the following solutions and see which one works for you.
Check your VPN connection. If you aren’t connected to your VPN, try reconnecting.
Disable security protocols on your WiFi network. Then, try reconnecting to your VPN. If this doesn’t work, make sure that you enable the security settings again.
Switch to another VPN server if you’re on public WiFi. Some public WiFIs have certain types of VPN traffic blocked, which may be the reason why you’re unable to connect.
If none of these solutions work, contact your VPN’s customer support. They may be able to guide you through the issue or fix it from their end.