The only relationship the horns and the ignition have is that fuse 8 (which only powers the horn) gets its power from the same ignition switch power leg.
This might mean that the ignition switch is bad. It also might just be coincidence. Check the power on fuse 8 and terminal 15 on the coil. They both should be hot and both are powered off term 15 at the ignition switch.
Bosch Master, ASE Master, L1
33 years MB technician