according to the definition of a sentence?
For the sake of a peaceful life you should accept the definition of "sentence" offered by your teacher, that will allow you to pass any exams you might be taking in English. However, "Hello." or "Hello!" (or even "Hello?") are all accepted as correct and complete utterances by speakers of English, and presumably by your English teacher. So you shouldn't let any definition of "sentence" get in the way of learning how to speak and write the actual language.
It's just a label used by those who study grammar to describe what they're talking about, and as you can see from the answers, not everyone is agreed what should qualify as a "sentence". Brian Hitchcock says it's a sentence (and therefore it's not true that every sentence has a verb). Soke says it's not a sentence (or anyway not a "complete sentence"), but is in the minority here.
So, your inclination might be to accept that "Hello." is a sentence, and reject the claim that in English every sentence has a verb. If you leave things there, though, there's a risk you'll conclude that any part-sentence is OK, and get things wrong. The rule that you need a verb is usually a good one, and that's why you're taught it.
If you learn that a sentence must have a main verb, then a full stop does not always indicate that what goes before it is a sentence. In that case the full stop is used to separate sentences, and also is used to separate some things that are not sentences. Fragments, for example.
Analysis and labels are not inherent properties of the thing they describe:
'Tis but thy name that is my enemy;
Thou art thyself, though not a
What's Montague? it is nor hand, nor foot,
nor face, nor any other part
Belonging to a man. O, be some other
What's in a name? that which we call a rose
By any other
name would smell as sweet;
So Romeo would, were he not Romeo
Retain that dear perfection which he owes
title. Romeo, doff thy name,
(William Shakespeare, Romeo and Juliet, Act II Scene II)