If they all dimmed at the same time, it's probably one of four things. 1. The bulbs are burning out all at the same time and the timing is a fluke. If you have any spare bulbs, change them out for the old ones. If they are brighter, problem solved. If not, move on to #2. 2. There is a loose connection in the box behind the switch. Get an electrician for this if you don't know what you're doing. If that isn't the problem, it could be #3. 3. There is a loose connection or damaged wire coming to the switch or going to the lights. Get an electrician to trace the circuit and see where the problem is. Most likely, if it's not a damaged wire, it's a loose wire in a juction box connecting the lights together in the ceiling. 4. You have too high a load on the circuit so there isn't enough power getting to your lights. Switch to lights that use less power or have an electrician change out the circuit with higher amperage wire and circuit breaker.