Keyboard split in half want back to one piece

Or just drag the two halves back together.

Look at the bottom right corner of the right half of the keyboard. See the button that looks like a Keyboard? Tap that and HOLD it. Select Merge or Dock and Merge. The first joins the keyboard halves. The second joins them and puts the keyboard back at the bottom of the screen.

Then go to Settings > Keyboard and turn off Split Keyboard. You'll never have this problem again.