First of all, empty the game cache

- launch DOFUS (you don't need to log in)

- click on the small key on the top right of the DOFUS window (options menu)

- click on the 'Empty the cache' tab

- a pop up will request confirmation, click 'Yes'. The game will relaunch automatically.

Then check the configuration of your Flash Player.

You have to inform Flash that DOFUS is not malicious software, which you can do by following this link. Add DOFUS to the trusted software folder.

Antivirus.

Some antivirus software block DOFUS. make sure that your antivirus accepts the game and does not treat it as indesireable software.

Finally, make sure that your internet connection is working correctly and that your hardware (modem/router...) are properly connected and configured. Then try to reconnect to DOFUS.