If you are unable to connect to your server:
First, check your server logs! If the game software is noting your connection attempt it is likely your game software itself is rejecting the connection. The server logs should go into detail on why you cannot connect (Mods mismatching, version mismatching, failures to load maps etc).
NOTE: Across most games you must be running the same version+build client-side as you do on the server. Steam games are known not to print any 'useful' errors when version mismatching occurs. You can contact support for further assistance if you do not understand the errors etc.
Try using the numerical IP from the Server > Overview > Access menu (Old Panel: Virtual Server > sFTP/SSH Access). Note: If you are able to connect with the numerical IP, but not your hostname... Make sure your DNS is flushed, or, change your DNS server (188.8.131.52, openDNS, GoogleDNS etc, there are many available online) to ensure your computer is retrieving up to date DNS records.
Visit the Virtual Server > Power Controls page and run a system update (this will check all the firewalls are working as intended and if not, should kick them into proper order!). Note: Public port checkers will fail if the software on the server is not designed to respond properly.
Make sure your game software is started to competition and awaiting player connections.