Yes, if followed by a comma.
No, "suddenly" is not a complete sentence. It is an adverb that describes how an action is performed in a sentence.
Suddenly.
Suddenly the boy whacked me on the head.
Suddenly, The door slamed closed
Suddenly, The door slamed closed
suddenly loomed ahead. Predicate is everything except the subject. The subject in this sentence is 'A steep mountain'
Suddenly she lashed out menacingly
Had
Suddenly, we encountered the lunch lady.
I can give you several sentences.Suddenly, the lights flickered and went dark.He suddenly began to cry.We were suddenly surrounded by pygmies.
The common noun in the sentence is screams.
A man was walking down the road when suddenly his sentence was interrupted by the earthquake.