My DD did this when she was also about 2. fell and knocked the front tooth back up into the gum. When it re emerged, it was grey, but it got a bit lighter as time went on. It didn't fall out or need to be removed, and her adult tooth came through undamaged. HTH
DD2 did this with one of her front teeth, she fell on the path in May and banged it hard. Dentist said it should be fine but over the past few months it has got more and more grey and is now wobbly.
We went to the dentist again today just for a check up and were told it will probably fall out soon. The dentist told us that hopefully her new tooth has not been damaged, but until it grows through (or doesn't grow through) they can't do anything. We just have to wait and see and she will probably have a gap for a while!
I did this when I was little. When my second tooth came through it had a small mark on it, like a bruise. It's not noticeable at all and is perfectly healthy. I wouldn't worry - I'm sure she'll be fine.
My DS1 did this when he was 2 and it took a long time for it to go completely black but it eventually did - then an abcess formed on his gum - 3 courses of antibiotics and it just kept coming back. He had to have the tooth taken out a month or so ago (at 5y 11m) and we just have to wait and see if his adult tooth is going to come in okay.
Ditto to all of the other Mum's posts. My Son fractured four of his top teeth in a fall at 2. We were told exactly the same by the dentist. At almost 7 they are still all there and fine - we suspect they will probably be the last milk teeth to fall out!! The good thing was it got him into an NHS dentist early and him having regular checks later than he probably would have done. Try not to worry.