So what do you do when your father walks into your room while not knowing your significant other is there, and catches you half naked?

Well today my dad got home super early, and came upstairs to my room. My SO was with me and we were not fully clothed. My father doesn't... Well, didn't know I was sexually active. He is big and scary. I'm afraid of what he will do or say to me. When he walked in he said hi. I said heyyy. What should I do? We're not close. Do I avoid him or try to talk to him? What do I do?!? Help. Please.