My fillings fallen out but it had a root canal will it hurt later?

YES: A root canal can fail two ways. 1 if the end (apex) isn't sealed 2 if the filling or crown is lost and it is exposed to the oral cavity, it will also cause failure. Endodontists recommend redoing the root canlal if it has been exposed to the oral cavity for more than 2 weeks it will eventually hurt.

It should not: If the tooth had root canal therapy successfully completed, then it has no nervous innervation so it should not hurt later. However in the long run the tooth should be restored [filled].

Get it fixed: Asap because a root canaled tooth is brittle and will need to be restored to keep the root canal treatment itself from failing.

It could: Not at first. However once the root canal is exposed to your saliva it could get reinfected. Some studies say it could take 30 days to get re contaminated and thus more prone to infection. If it gets reinfected it could cause you pain. Lastly, if the filling is out it may mean the hole in the tooth is to big for just a filling and would be better off with a crown.