1. Press CTRL+SHIFT+ESC simultaneously on your keyboard to open the Task Manager.
2. In the Processes tab, select and close the UbisoftGameLauncher.exe process.
3. After closing the process, uninstall the Uplay client from your PC. You can find the uninstaller for Uplay in C:\Program Files (x86)\Ubisoft\Ubisoft Game Launcher if it is installed to the default directory. The uninstaller is named Uninstall.exe.
4. Now you'll want to download and install the latest version of Uplay from http://uplay.com/ by clicking the Download Uplay For PC button located near the center of the page.
5. Now that Uplay is reinstalled, try launching your game. Ideally, it should not prompt you for an activation key again.