I have accidentally given students assignment guidelines that conflict with the course coordinator's. How I can rectify my mistake?
Talk to the course coordinator immediately and coordinate your next steps. 

Apparently, there was a serious breakdown in communication, and 


  the course coordinator didn't provide any guidelines whatsoever


suggests to me that you are not the only one to blame.

Whatever your next steps will be, they need to be aligned with the coordinator and his expectations. You may be able to extend deadlines, adjust grading rubrics or something else.

Next time, make sure you know what the coordinator wants. (And the coordinator should make sure, too.)