Have you asked her about it?
Don't accuse or confront, just gently say, "I really enjoyed it when you gave me oral sex, and felt really close to you then (or whatever). I was wondering why you stopped. Was it something I did?" Don't do it during sex. Do it during a quiet moment with just the two of you.
Then be open to hear what she has to say, and don't get defensive. This is a discussion, not a heated debate or fight.
There must be some reason, but we don't know - maybe she doesn't like it, or doesn't like something you do during it, or she feels uncomfortable, or you aren't reciprocating, or you expect it to be like porn. Maybe she's exhausted from working, kids, being a wife and it's too much - I don't know and the list of possibilities is long.
After 15 years of marriage, though, you should be able to have an open, honest, gentle conversation about sex. Maybe now is a good time to explore why that isn't happening.
I'm going to say talking about it is the best thing to do. But maybe she was always kind of doing it just to please YOU and she's ticked off at you deep down for something so just isn't interested in it now. Are you getting alone?