I’m in exactly the same situation OP with my best friend of 20 years. Only I know what I’ve done, not attend a party with her, her husband and 4 other friends because it was a 6 hour drive there and back, I had no money, was exhausted after a long week and my partner had to work so I’d have been the only ‘single’ person going (when initially it was just meant to be a girls night out!)
I apologised profusely but explained I really couldn’t afford it and it was a long drive there and back for me (it was only a 1 hour drive for her because she’s recently moved) for a few hours at a party. I NEVER bail on plans so I thought she’d understand but she was quite irate and rude. She then said it was fine but she has barely spoken to me since and this was 2 months ago.
I miss her but I REFUSE to grovel or apologise again. It would just confirm to her that she is in the right, when IMO, she is not. There were 5 other people going, it's not like I left her to go on her own (which I wold never have done!)
In your case though, when you really don’t know, I would have to ask what it is I’m meant to have done! It’s very childish of her to cut you off without having it out with you first. I'd hate having to ask as I'm quite proud and stubborn, but then again, it would eat me up not knowing.
Try and call her, or go around and see her? If you're not comfortable with that, how about an email? Though I think i'd prefer a Whatsapp or FB message personally as at least then you know whether she's read it!