You have X chillers per aeroplane, varies from type to type and airline to airline.
Eg: Qantas have more chillers than BA usually.
If one or more break down the cabin crew can sometimes shift carts to other galleys (like moving from one frig to another) The carts "plug in" to the cold air supply.
Having said that I have seen four chillers break down on one 15 hour flight- thank goodness engines are more reliable!
If your meals were warm perhaps the cabin crew didn't notice the problem with the chiller till too late- or perhaps they were too lazy.- think how much work they'd avoid...... moving AND serving!