While there may be a medical explanation for your inability to have an orgasm, chances are the cause is in some way cultural or psychological. You may be unable to climax because somewhere along the way you concluded that sex is a shameful topic, or (because you've had a negative sexual experience in the past), you're uncomfortable with your body or you're just not familiar yet with what turns you on.
Who to talk to
Your first stop should probably be a sexual health doctor or sex therapist, who will likely encourage you to explore your own sexual response. "Have you tried a vibrator?" asks sexual medicine specialist Andrew Goldstein, MD, an associate professor at George Washington University. "I tell women they need to masturbate!" says Hilda Hutcherson, MD, an ob-gyn professor at Columbia University.
What to find out about your own sexual response
Do you prefer a light touch or a firmer one? Do you want your clitoris to be stimulated or your vagina to be penetrated, or do you need both at the same time? Despite the impression you may have gotten from romance novels, many women don't have orgasms from vaginal penetration alone. A 2003 German study found that 70% need direct clitoral stimulation in order to achieve orgasm.
Once you figure out what makes you climax, the next step is communicating that to your partner. Working with a certified sex therapist can be helpful, in order to talk through any fears or shame you might have regarding your sexuality.
While most cases of anorgasmia (the medical term for not being able to have an orgasm) are psychologically or culturally based, there can be medical reasons for the problem, too, so it's important to consult a doctor, especially if you used to have orgasms. Anorgasmia can happen for all the same reasons as low libido: reduced blood flow to the genitals due to disease or medication, hormone imbalances, or the use of medications that dull sexual response.