IR controlled light dimmer
What you've described (without source code and/or a circuit diagram) sounds like you'll probably be using interrupts at least for the zero crossing detection. And probably the IR receiver too. The IR receiver is going to be your problem since the zero crossing is a very quick flag set and you're out of the interrupt. 

On the other hand, the IR detection is going to take time, and if you're doing THAT in an interrupt you're going to be missing dozens of crossings. 

To mix the two you can leave the zero crossing in it's own interrupt, but you're probably better off handling the IR in the loop via a state machine. 

Since you say that neither are working, I'm going to hazard a guess and you're using the same interrupt or badly implemented them (possibly a timer and interrupt clash) and until you post some code people here aren't going to be able to help you any more than that.