It might be hard to figure out what is going on. In our case, a well-meaning and usual kid activity caused the problem. My son was unusually verbal and could tell us that he was having horrible dreams about lions after a fun trip to a zoo. He hated lions for years. We had two cats so it wasn't about that. I never did figure out exactly why the lions were so terrifying because they barely move at the zoo.
I'd find a gentle bedtime book and see if that helps. Evenwhen kids have no words or very few (my other two didn't have any until almost 3 so we have a wide range of experience), they can understand and get scared and also feel comforted by stories and experiences. They still won't look at the Sendak books or the Dahl books. Something lovely to dream about might do the trick.