Why do i get no sound from Traktor Pro?

I have recently bought Traktor Pro and am using a Numark omni touch mixer. They both worked perfectly then i turned my PC off and re booted it this morning to find Traktor works (plays music) but you cant hear the sound, you can only see the music playing through on the screen. All other windows sounds work on my pc and i can play music on media player perfectly, its just traktor which is the problem.... Any ideas?