Did you fall out of love?
Don't you get satisfied when you're having sex?
Does your current lack of interest concern just your boyfriend or is it a general feeling?
A lack of sexual interest can be conditional on the personal situation, such as a stressful job or family problems, or it may reflect problems between you and your partner, or some learned experiences from your past. In those cases, it can be cured by a psychotherapist, preferably one experienced with sexual problems.
It can also be due to some basic natural variation. It is established that people have different needs for sex. Some want sex once a month, while others want it several times a day, and some do not want to have sex at all. This natural variation can't be labeled an "illness" which can be "cured."
It is a good idea to find a partner who is compatible within this area ,so as not to create any problems. It can, however, be difficult to find a man when one is not at all interested in sex, since it's the erotic attraction which helps people find each other in the first place. I have met several young men who have been disappointed in their partner's lack of sexual interest. There seems to exist a difference in gender. However, I have also met women who haven't been interested in sex with one man, but who feel sexual desire return when they met a mate who is more compatible.