Lamp doesn’t light up despite power?

I wanted to move my ceiling light in the living room by about half a meter. No sooner had I done it than I started testing. The light no longer lights up, even though it did before. I double-checked everything to make sure it was wired correctly (including its original configuration) and that there was power.
Three cables come out of the ceiling: red, black, and gray, and blue and brown on the lamp. According to the electrical tester, the red cable is receiving power. Even when the lamp is plugged in and I check the socket, the electrical tester lights up. The fuse is still in the socket, and I also tested the lamp in another room, and it worked.

So what could be the mistake?

I also read something about the switch in a forum. Mine is a bit stiffer, too, but it still worked before.