The morning after the LORD spoke to him in the night, what was young Samuel afraid to do?

Answer: Tell Eli the vision

Samuel lay down until morning and opened the doors of the house of the LORD, but he was afraid to tell Eli the vision.

“Samuel lay down until the morning; then he opened the doors of the house of the LORD. He was afraid to tell Eli the vision,”— 1 Samuel 3:15 (BSB)