AcornFairy That does sound very hurtful. If you think coercive control is a possibility (maybe made worse by lockdown), I would try to find out, but very carefully to avoid her husband taking anger out on her because of you contacting her. I don’t think Sellotaping an envelope would be enough. Maybe a phone call when you know he will be out (after lockdown if necessary)?
How close friends were you? Does she live far away?
Other possibilities (as mentioned): mental health problems (made worse by lockdown), and/or there was something she wasn’t happy about before lockdown and it feels worse to her now. If it turns out to be one or both of those, you could try to leave the door open to further contact, in case she feels differently later. If you think you might want to be friends with her again, that is. Maybe a text saying something like, ‘Sorry to hear you feel that way. If you feel differently later, do get in touch.’
I agree with people who have suggested asking people who know her if she is all right (if you can).
If you find out it wasn’t coercive control, you could consider if there was anything you did she might not have liked, or signs of anything wrong between you. Or of her dropping other friends as some people have described.
If she was unhappy about something, it would have been more fair to talk to you about it before ending the friendship. Sending best wishes.