Audio tracks knock themselves out of sync ?

Hope someone can help with this !

I’m running Cubase 6 on an iMac (Yosemite).

Had absolutely no problems in the past but working on a track today, I exported MIDI tracks and when I went back to listen to the track again, all the audio tracks are out of sync with each other. Never come across this before !

Don’t know if exporting MIDI tracks has anything to do with it and I don’t think I’ve touched anything else.

Any ideas ???

Thanks in advance