I used the word reoccur in a paper and my teacher said it should have been recur. Can you tell me the difference?

Recur and reoccur mean the same thing -- to happen again. But careful writers make a fine distinction. Reoccur is used to mean to happen one more time, while recur indicates a repetition more than once, usually according to a schedule or pattern:

Her terror reoccurred when she saw her attacker in the courtroom.
The recurring movement of the tides changed the shoreline.