I had a very similar episode last week, the return legs of my DUB-LHR-SFO disappeared from MMB. I assumed a glitch but after a few days decided to call BA. It turns out that the return legs had been cancelled yet I had not received any notification, nor did they show as cancelled in MMB. Reason - the departure time of the final LHR-DUB flight had been moved forward by 5 minutes which thenleft a minimum connection time of less than 60 mins so the system automatically cancels both legs. Thankfully got the flights rebooked but still pushing BA to explain why I was not notified.
Call BA to check the status of your flights.