Your toenail fell off but why? Typically a toenail falls off because the toenail is dead. You either bumped or bruised it and the toenail died. This is not a horrible thing. A toenail will grow back within 6-9 months. You are better off with no toenail, than a dead toenail. A dead toenail is a reservoir of infection, as bacteria will attack and colonize the dead toenail. If you have a dead toenail and it is barely hanging on you are better off having the toenail removed, either by a Podiatrist or a good yank. Dead toenails serve no purpose other than to become infected.
Are there any risks with the new toenail? There are two risks really. One is that you will acquire a fungal infection and the nail will grow back with a fungal infection. The chances of this are low, assuming the nail was not infected before. But, if you are worried that the toenail will grow back with a fungal infection simply buy a topical anti fungal for toenails, and paint it on as the toenail grows back. Here are a couple recommended:
The other risk is the Nail matrix is damaged. This is bad, now it is hard to do this with a bump or bruise, but not impossible. The more common way to do damage to the Nail Matrix is a failed nail surgery. There is no way to fix a damaged nail matrix, your toenail will never be normal. You have two options: just have an odd toenail or have a permanent nail matrixectomy to the have the toenail removed permanently. Here is a video that discusses whether you should have a permanent toenail matrixectomy: