This is a fragment. In order for something to be a sentence there must be a subject that does the action/verb. In this sentence something or someone went to the lake, but we don't know who or what (is it "I", "we", "they" or "it"), therefore it is a fragment
This would be a "fragment" "Went to the lake' lacks detail and it's short which would make it a sentence fragment because it's a portion of a sentence. So, We all went to the lake for a swim' would make it a sentence.
