It is a sentence.
The man was frightened to see that his son had killed himself!
She had never been so frightened before.Mummy, James frightened me again.
I was so frightened in the haunted house, it really scared me.
Frightened is only capitalised if it begins a sentence, or is part of a title.
He was frightened badly. So I talked him down.
The verb in this sentence is "peered." It describes the action of the scary face looking out from the window.
Macabre movies make me frightened.
The lion that looked like a beast frightened me.
The correct capitalization for that sentence is: "Neither the dog nor the cats were frightened by the delivery truck from Hazel's Florist."
That sentence doesn't contain a direct object.
The bird was frightened away by the noise. Are you frightened by swarms of bees? (One swarm can ruin your whole Saturday night!) A spooky carnival ride is one place where people enjoy being frightened.
A ghostly apparition frightened Scooby Doo.