Monsters?

Will is now very afraid of monsters, and we have no idea why. I don't know how to handle it. I keep trying different tactics, but none seem to have any effect on him. And everytime he tells me the monsters are going to get him, and I have to come up with some response, i freak out. What if I choose the wrong reaction? Will it scar him for life (or childhood)? Should I ignore it and tell him he's fine, the monsters won't get him, there are no monsters (we have tried all)... Or should I proactively teach him how to defend himself against them (i taught him how to beat them up, and to tickle them). we turned on all the lights and scared them away. we locked them outside so they couldn't get in..... Nothing is working! If you have any advice, send it my way. or refer me to someone who can! thanks!