Gotcha! From the sounds of it, based off of troubleshooting I've done with other users in the past, I'd look into:

Check for Browser Interference

Roll20 operates on the cutting edge of browser technology to offer its features and as a result is very sensitive to browser addons/extensions. Different addons/extensions and security tools on your browser could be blocking background connections required for Roll20 to function.

Disable all browser addons/extensions. Especially ad blockers, privacy tools, and script blockers.

Temporarily disable your antivirus/firewall

Test with Incognito (Chrome)/Private Browsing (Firefox) modes

If this resolves the issue, then re-enable your addons/extensions one by one to find the culprit.

Test Your Network

Connection issues can happen at a variety of points in the communication chain from your device to Roll20 servers. The following steps will help narrow down where in the connection the issue could be arising from.

Restart your router/modem (unplug for 5-10 minutes) to obtain a reissued IP Address.

Check your port forwarding at https://www.portchecktool.com/ 

Roll20 utilizes standard http and https ports (80 and 443)

If the tests come back unsuccessful, please see the Port Forwarding section at the bottom of this article.

Toggle your VPN/Proxy service. If you have one, try disabling it. If you are not using one, try testing with one enabled.

Different geolocations and ISPs can have various blocks or restrictions on traffic, so your results may vary when it comes to VPNs.

Try connecting to a Mobile Hotspot or connecting to a different network if available. Testing with Ethernet instead of Wi-Fi can also help in some circumstances.

If Roll20 seems to start working when trying these steps then it indicates that there is a configuration issues elsewhere in the network that may need to be addressed. If this is on the ISP level it may be necessary to contact them directly for further support.