[prettylily]

There is a condition where your vagina will close itself off. Usually from anxiety, I think.

http://www.webmd.com/women/guide/vaginismus-causes-symptoms-treatments

Have you or your boyfriend had sex before? And have you tried the lube or lots of foreplay before hand? If you're not very slick down there and haven't had sex before, it can be difficult to get his penis in.