Happened to me. We are stranded in USA and my MMB was greyed out after flight was cancelled. Couldnt get through to BA and so i rebooked and bought another ticket. So far i am £1000 each out of pocket.

I called BA again to at least cancel my original flight and got through within 2 mins .

Ba told me that as i have rebooked and not done it via the old booking i have to talk to my insurance co regarding reimbursement.