I struggled with this with my DD for a while too before I just accepted that she got herself there, she can get herself back and she's happy that way. I would continually roll her onto her back and all that happened was we both got disturbed sleep and I had a grumpy baby who ended up sleeping on her tummy anyway.

The thing I did find helped me was a movement sensor pad under her cot mattress so I had the reassurance that although she was on her tummy, she was still breathing, which meant I could finally get some sleep too! If your DD has head and neck control she'll be fine - and she obviously does if she's happily rolling away