I've been there. I chased answers for a long time. I was depressed, cried a lot, asked mutual friends what was going on but she lied to them and said everything was fine. I had to finally let her go so I could move on with my healing. I still to this day get anxious when I see her at a mutual friend's gatherings.

Over the years I've done a lot of reflecting and have come up with a couple theories that make sense to me, but will never know for sure. It really tore me up for a long time, and would have continued to do so had I not let her go.

In your case maybe it's not as severe. Perhaps give her some space and try again, let her know you want to make things better but cannot do so unless she tells you why she is upset, but recognize there may come a time when you have to move on. (It took me years to get there if I remember correctly).

Good luck with your friend, and I'm sorry.