Is A great , big monster jumped out   a sentence ?

Dear Student,

Your query was not clear to me. Could you please mention the sentence clearly for us to be able to assist you further on the same?

However, if the line you mentioned is: A great, big monster jumped out', it is a sentence but conveys a very incomplete meaning. The sentence is in a transitive form and thus needs to be completed with reference to the fact of 'from where did the monster jump'.