Sometimes a landlord, letting agent or lender sends a payment back. When that happens the money is returned to your rent and mortgage balance. It stays yours, and you can withdraw it at any time.
The most common reason the money may be returned is the payment reference. Lenders in particular use it to match the payment to your account, and some will return anything without the exact reference they expect.
Two things to do:
Contact your lender, landlord or agent and ask what reference they need, and whether they accept a standing order at all.
Update your setup with the right reference. Go to your rent and mortgage screen, tap Manage, then Edit.
Your payment is still owed for this period, so arrange to pay it another way in the meantime. Get in touch if you'd like a hand.