I had this problem too. It lasted about 2-3 weeks and then DD learned to roll herself back. She also learned to like sleeping on her tummy but that doesn't worry me as I know she can move her head around and roll back if she wants to.

At first I tried to keep her from rolling over by using some little roll things I bought at a baby shop but she learned to roll on top of them and would then get stuck which upset her more so I stopped using them after a couple of days.