Why do we use a s' in this sentence:
La fenêtre ne s'ouvre pas !
Is it because of the two vowels or for the type of verb?
« Is it because of the two vowels? »
S'ouvrir would be contraction of se and ouvrir.
Verbs that have se (or s') in front are called reflexive verbs.
They are about what an object does (or in this case doesn't) do to itself.
So "La fenêtre ne s'ouvre pas !" means, "The window doesn't open itself.