In addition to the questions thatquietgirl asked, could you be depressed? Depression can completely kill your sex drive and make it not fun anymore. If you think depression is even a remote possibility, try taking the Beck Depression Inventory. It includes a question about sexual desire/health, and it gives a general severity score instead of a complete yes/no answer. Either way, you should talk to a doctor about this, because such a loss is not normal.
You need to tell your boyfriend about the problems you're having if you want to fix them, even if it is depression killing your sex drive. He won't be able to help you at all if he doesn't know something's wrong.
Has it become routine? Are you getting enough foreplay? How is your out-of-the-bedroom intimacy? Do you guys let each other know you find each other desirable through little touches throughout the day? How is the relationship otherwise? A lot of these things can affect how things feel for a woman. I'm not sure what else could cause a drastic change like that unless you had some sort of damage down there at some point?