When BMI were being merged in to BA I took a flight MAN-LHR-MAN to attend a GGL event. The MAN-LHR was a BA flight operated by BMI. All was fine until that evening when I noticed my flight back to MAN was showing as cancelled on the BA App.

I thought nothing of it assuming the LHR-MAN had gone tech and called the GGL line to rebook. I was told I'd no-showed for the MAN-LHR and therefor my booking had been cancelled. I had actually taken the MAN-LHR segment but it did not show on BA systems. It took over 30 minutes to get my return flight reinstated.

In the OP's shoes I wouldn't bother even trying, BA will have cancelled his booking as a no show and unless there are compelling or compassionate reasons to do so, I suspect there is no chance of the ticket being reinstated.