First, check to see if Webex is up and running properly. You can check https://status.webex.com to get the latest status. Or, on Webex for Windows and Mac, you can also use the Health Checker within the app itself.
You should also make sure you don't have anything on your Wi-Fi network, such as a firewall, that might be preventing you from using Webex. If you are using Webex for iPhone, iPad, or Android, try switching to your mobile data connection. If the app works when you're connected to mobile data, then the Wi-Fi connection might be blocking Webex traffic. If you're using Webex at work, report this issue to your administrator.
If the problems persist, you can contact your company's support team, or open a support case for Webex.