In addition to the above recommendation I would try a couple commands.
Try NSLOOKUP
www.dropbox.com
Then try
NSLOOKUP
www.dropbox.com 8.8.8.8
The IP addresses you get might actually be different because of all the virtual hosting. What is more important is do you get a IP address, especially from the first one.
If the first one give you a IP see if you can ping the IP.
Generally if you do not get a IP at all it is a DNS issue. If you can not ping the IP then either a firewall or something is blocking it.
If everything appears fine then it is more like some browser issue because a ping command show you have basic communication with the server. Note there are some servers that do not respond to ping but dropbox does.