I've just changed the fixture, black to black, white to white and ground to ground, but there is a red wire coming from the junctionbox. When I turned the breaker back on, the light works, but not the switch. Obviously, the red one controls the switch, but which wires do I attach it to?

That red leg is the hot wire from a 3 way switch. It gets connected to the fixtures black along with you other black. Somewhere there is a second switch if I am reading this right. Let me know about that second swicth please

That's what got me confused, because there isn't a 2nd switch. It was a ceiling fan before, with a light, so you could turn on both with one switch. I'm going to try to connect to the blacks. Don't worry, I always turn off the breaker!

Tell us how many and what color wires are in the ceiling box. Tell how many and what color wires are in the switch box.

Hold on a second!

If the switch controlled both the fan and light, than more than likely the black from the switch went to the fan's black. The red from the switch went to the light's blue wire.

You need to make sure you cap off the red conductor. Otherwise you have just found a place to start a fire!

I agree, the fan and light each had a swicth but now that has been undone and only one switch is left. That switch is sending power via the red or the black. Test and find the hot one and attach the fixtures black, cap the other wire with a wire nut.