To resolve this issue, you will need to manually install or reinstall the Uplay software (which handles the online authentication in the game). Please note that this software MUSTbe installed to play the game, and cannot be uninstalled or disabled.
1. First, you will need to uninstall the "Ubisoft Game Launcher" or "Uplay" from the 'Add/Remove Programs' (Windows XP) / 'Installed Programs' (Windows Vista and 7) icon in your computer's control panel. It's possible that the "Ubisoft Game Launcher/Uplay" software will not be listed if you have already uninstalled it, or the game. If so, you can skip this step.
2. After uninstalling the software, you will need to manually delete any leftover files in the C:\Program Files\Ubisoft\Ubisoft Game Launcher (32-bit versions of Windows) or C:\Program Files (x86)\Ubisoft\Ubisoft Game Launcher (64-bit versions of Windows) folder.
NOTE: Please do not skip this step, it is very important. If this folder does not exist, please go to the next step.
3. Try resetting your HOSTS file by following the steps in the following Microsoft article.
4. Reinstall Uplay PC from HERE.
Once installed, you should be able to start the game!