It's possible the multiplayer servers are temporarily down: check the status here - USA, East (Realtime).

In order to connect to multiplayer, make sure your firewall or network are not blocking your connection to the following servers.

• parseapi.back4app.com (port 443)
• ns.photonengine.io (port 4533)

If you are still having trouble, ensure the ports listed on this page are not blocked.