Just as per the title really.
Long time old friend has had intermittent mental Heath issues since teenage years (depression).
She has been uncharacteristically quiet and reclusive for the past month and has missed/is about to miss some social occasions that she'd usually be front and centre for.
All she has said (breaking a non-responding silence on phone, email, WhatsApp and social, which she is usually all over) is that she wants to be left alone to work, see her family (as in her parents and sisters families, who she's extremely close to; she's single and hasn't had a long term exclusive relationship).
I am guessing at a breakdown but she has said she "doesn't want to go into" what's wrong, so I don't feel like I can ask.
What does a very bemused good friend do in this situation? Her actions are impacting other friends who feel at best ignored and at worst aggravated. She owes people money (not huge amounts, under £100 to a couple of people) and hasn't RSVPd or booked accommodation to a wedding 4 weeks from now.
How can I help her or should I be leaving well alone as requested?