This is a very common problem, but the good news is that you can expect in the vast majority of cases for the toe nail to grow back over time without any problems.
Unfortunately, this can take quite a while. In most cases, if the toenail has fallen off completely, it probably takes several months for the nail to grow back in completely. There is not much you can do to hurry along the process, other than to keep the tender skin under the nail well protected.
Occasionally, if there has been damage to the nail matrix, where the new nail material is manufactured, or to the nail bed under where the nail used to sit, then the nail may not grow back in normally. It is best to have your primary care doctor
look at the damaged toe to make sure there is no evidence of scarring or other damage that might prevent normal growth of the nail. They can also make sure there is no evidence of infection which also might delay healing.
Good luck, and please see your doctor for more information!