Temporary filling fell out after root canal, what can i do?

Get it fixed!: Root canal treatment must be followed up with a definitive restoration of the tooth. A leak into the root canal filling could cause a failure of the root canal treatment. Also, teeth that have had root canal treatment are more likely to break and must have a strong final restoration placed to help prevent this.

See your dentist: In general a temp fill is placed after a root canal, it will last for a couple weeks. Meanwhile you should make arrangement to go back to the treating dentist to get a permanent restoration ( build up or post and crown etc...) you should not leave the tooth unprotected as the bacteria will seep back into the root canal and cause it to fail again.

Temp filling: Some drug stores sell over the counter temporary fillings that you can try. Since the root canal seal is far below the surface of the tooth, there is little danger of infecting the tooth from the lost filling assuming you have an appointment in the near future to restore the tooth permanently.