Replaced light fixture, now 3 way switch doesn’t work
Sometimes a NM cable is not fastened down along its length, either because it was fished through an existing wall (old work) or because the builder just laid it between the attic joists (poor work).

If the NM cable that you moved is not thoroughly fastened down, and if the strain relief at the other end is inadequate, then you probably moved the wires or wire nuts around inside the box at the other end and knocked one of the travelers off its terminal.