This can have several causes. Please try the following solutions:
1. Check if your device is connected to the internet (cable or WiFi)
2. If you are on a company network (e.g. via VPN), this often blocks the connection to the webinar. Preferably use your own WiFi signal, your own internet connection at home or, if you are at work, your mobile phone.
3. Try to refresh the page or use the 'refresh arrow' in your browser
4. Try using a different browser such as Chrome or Safari
5. Are you sure you have updated your browser to the latest version?
6. Maybe your device is a bit older and might be incapable to handle the technology we use. Try another device (like your newest smartphone).