What is the reason for vagina pain?

Multiple diagnosis : Could account for vaginal pain. These range from infection (yeast, STDs, many), to a condition called vaginismus - in which vaginal pain is experienced on attempts at penetration. Other explanations could include minor abrasions after sexual intercourse of other types of penetration, or even vaginal dryness may result in pain. Please see your ob so that the source of your pain can be identified!

Need more data: There are a number of reasons for vaginal pain, but some more data will narrow down the answer. For example, a constant pain will generally have a different diagnosis than pain that only occurs during intercourse.

Hard to say: Unfortunately with so little to go by info-wise, it could be many things. It would help if you could provide where the pain is(deep vs superficial), any discharge, any odor, any rash, are you sexually active, does it hurt all the time or only on sex.