Chances are, it was something you said.
Thinking back on similar situations, I find that the offenses which can cause women to ignore their former male friends may seem minor.
If you criticized her favorite TV show or burped so loud that her kid sister cried, you can probably find out about this from her and patch things up.
Along with most of the others who answered this question, I want you to ask her, and if she won't say what is wrong, ask her friends and relatives.
It's possible that you didn't do or say anything wrong, and her family or BF is pressuring her to see less of you, because they are protecting her from an unwanted, possibly romantic, association.
That's not a minor problem, and you may need to think long and hard about whether you can still stay friends with her over her family's objections.