Assuming your ticket is a return on the one booking:
LOS-LHR-ABZ-LHR-LOS
If you miss LHR-ABZ, then both the ABZ-LON segment and LHR-LOS segment will be cancelled; i.e. all sectors after LHR-ABZ.
You should have booked the two legs separately originally
LOS-LHR-LOS and LHR-ABZ-LHR. That way, at least you would only loose ABZ-LHR only with your LHR-LOS being safe.
Sorry you had to learn the hard way. Many have been caught out in a similar fashion. These are currently in the Terms and Conditions. There are moves to legislate against this although that will be a long time off