Press and hold the button with a keyboard symbol on it (it's next to the numbers button, which is next to the space bar. You should get a popup that looks like this:
If your keyboard is split AND floating, you might see this:
Regardless of which one you see, pressing the Dock line will put your keyboard back where it belongs.
You can also use that key to make the keyboard undock and split, or you can just "pull" it apart....