Make sure you have an active license and it's not fully consumed. Then make sure you don't have any license rules, or if you do, that the access key matches the one in the license rule.
Next would be to double check your agent settings for the correct controller host, port, ssl, access key, application name, tier name, node name. It's easiest to verify this by running the AppD configuration GUI under the windows start menu.
You may have a firewall blocking outbound traffic.