Since it's an Android launcher, the simplest thing to do is to reinstall the application. Open Play Store, then type Launcher3. Tap on it, and it might say that is installed, therefore Open instead of Install and Uninstall next to Open. Uninstall it then install it again and press the Home menu. Choose Launcher3 and it should start normally.
Or go to Settings > Applications > All > Launcher3. Tap on it then Clear Data and Clear Cache. Restart the phone and it should work.