Re: A sudden fear of the floor
The other folks here have aready provided some good suggestions. Here are a couple more.
Tell them forward rolls are about getting away quickly & safely. I was going to say tell them they're NOT about falling but that's negative programming ; )
Have them look at their obi knots when rolling.
Trace a line (with your hand) from their pinky finger tip up their arm, across their shoulder, down their back to the opposite hip, prior to having them execute the roll. Be sure to keep contact with them at every point on the line. This tactile programming will help them roll along the proper axis of their body. We even do this with beginning adults in our dojo.
If they're really having trouble you can cradle their head while facing them in seiza (not for standing rolls).