For supersonic acceleration (Mach 1 to about Mach 1.7), Concorde needed all four reheats (too much asymmetric thrust with three).
So if for whatever reason one of the reheats didn't light at the start of the take-off roll (which occasionally did happen), the take-off was abandoned, and the aircraft would return to the stand.
Such a rejected take-off was a pretty straightforward affair, because the aircraft would usually not have reached more than about 50mph.