It's possible that your computer still thinks the launcher is running, especially if the game crashed or you had to shut the game down through Task Manager, etc. Look in your system tray (i.e., the group of icons that are in the corner of task bar by the clock on the bottom of the screen) to see if you see the launcher icon. (You may have to hover over the system tray or click the little up arrow to see all of the icons, depending on what version of Windows you have.) If you see it, just right-click it to open or to exit and then restart.
If it's not there, open Task Manager and look in the processes tab. If you see something like sims3launcher or sims3, right-click on it and select end process.
If you don't find it there, then the only other thing I can suggest is the standard help desk reply of "did you try turning your computer off and turning it back on again?"
Good luck!
