It sounds like when your daughter fell she injured her front tooth as well. Sometimes the tooth will become discolored because of the bleeding that occurred within the tooth. If that is the case, the gray color should go away within a few weeks, and the tooth will look totally normal. It should not fall out prematurely. If the tooth stays gray, however, then it likely means that the root of the tooth was disrupted, and that the tooth is now "dead". The tooth will be permanently discolored or gray, and it will be more likely to come out early. It is also at more risk for getting an infection or abcess.

There wasn't really anything you should have or could have done to change the course of this. Fortunately, this is still the primary or "baby" tooth, and it is likely that her permanent tooth will be fine. You should take her to see her dentist, however, so that it can be evaluated. The dentist may want to take x-rays to confirm that the permanent tooth is fine. I would make sure that you try and prevent her from injuring it again, as any further injury will likely disrupt the root.