I have gone through this, around the time I started premature peri-menopause. I could have orgasms, but not whenever I wanted, and they were hard to get.
I saw my GYN and he first tried Viagra, which didn't do much (made me horny as hell, but I don't know if it helped the problem, as the problem was intermittent) then when that didn't work (I couldn't afford it, as my insurance wouldn't pay for it, "We only pay for it for men." Assholes.) my gyn put me on an estrogen creme used intravaginally and after about a month of constant use, things got better.
Is there any chance you could be in peri-menopause? Are you taking a new antidepressant or a pain medication or blood pressure med? Any of these could cause the problem. I'd talk to your GYN and see what he or she has to say.
And, yes, I was bitchy as hell. I get very frustrated when this happened. In fact, it was the reason My Man and I started using sex toys!
They certainly helped, too.