I had a tooth that i got a root canal in and it finally fell off. what should i do?

See dds: You probably did not get a crown put on and the root canal broke. Hopefully you can still save the tooth.

Get it sealed up: The piece of the tooth or restoration that fell out has likely left the actual root canal treated area exposed. This will cause failure given enough time. The rule of thumb is generally 2 weeks is the point of no return, the tooth needs to be retreated at that point. Before that, the odds are good that the tooth can be restored with out re-treatment.