You tripped the RCD
RCD is British for GFCI. I'm not sure if there's a separate German phrase for that.
Standard European practice is to have a "whole house RCD" protecting the entire house. This is normally right before, or right after, the main circuit breaker. However it is also common for the RCD to be combined with the main circuit breaker.
What is an RCD/GFCI? A better question is "how does electricity flow?" It flows in loops. Electricity goes out one wire, and comes back another wire. And the currents in both wires are always equal and opposite, since it's a loop. So what is an RCD? It is a device which monitors current flow on neutral and all hot wires. It wants to see that the sum of the currents (counting for direction) is exactly equal. i.e. cancel each other out.
Once all the normal currents cancel, what is left? Residual current. That means current going one direction, and disappearing / coming back ??? not through the normal wires. ??? Arcing to earth??? Shocking a human ???
So the RCD trips when that happens.
Since your light switch wiring is what changed, 99% chance your light switch wiring contains (or contained) the fault.
One thing that will get you is "smart switches" (especially cheap Chinese) that need neutral to power themselves. And the old switch wiring is a "switch loop" with no neutral. So the temptation is to wire the switch neutral to safety earthing. Now the switch's own power comes out the hot and returns on earth. Earth bypasses the RCD, so the RCD correctly sees it as residual current/ground fault. (ground=earth).