There could be numerous reasons for this. Start with doing a "tabadmin status -v" and verify that all processes are running.
Then, verify that the gateway is listening on port 80 or if you're running SSL, then port 443. Perhaps do you have other apps that are blocking this port? If port 80 is blocked, then Tableau Server tries to use port 8000, so add :8000 after your URL and see if this works. You can check ports by going to a command prompt and doing a netstat -abon. This will list out all the ports that are established / listening and what executable has it.