This can be caused for several reasons:
1) You might have made a mistake in a portion of your server address.
a) you entered the wrong ip to the server you are trying to connect to.
b) you entered the wrong port to the server you are trying to connect to.
c) you have a typo somewhere in your server address.
2) Your Internet provider or firewall is filtering TeamSpeak network traffic.
3) You were given a DNS based server address and your DNS services are not working.
4) Your Internet connection is down or so slow that a connection cannot be established.
5) The server you are trying to connect to could be offline.
Typical solutions to the above referenced items:
1) Check for typos in the IP address, port #, or spelling of your server address.
2) Contact your Internet provider to determine if TeamSpeak network traffic is being filtered or if you know how to do so, try reconfiguring or disabling your firewall to allow for TeamSpeak network traffic.
3) Contact your server administrator and request the server address in IP:PORT format instead of its DNS counterpart.
4) Make sure you have Internet access (try using your web browser). If your Internet connection is down (or too slow), contact your provider for assistance.
5) Contact the server administrator for assistance.