I just had a root canal and the temporary filling at top of tooth fell out. Should I go back to the dentist?

Yes. Ask your dentist about using a bonded core instead of a temporary filling. This will not dislodge and a crown can be placed over it and the root canal will stay sealed and protected.
Yup. The temp filling is used to keep the tooth clean and free of bacteria. With out the temp in place the tooth is going to get reinfected over time. Call your dds and get it checked.