Hi, how long have you had your tank? Has it been cycled, have you tested your nitrite/ammonia, what is your temp?
I am afraid guppies don't do so well unless your tank is quite matured, they are sold to people and told that they will be fine as starter fish or that they are the next ones to go in as guppies. Do they have any spots, irratic breathing, any damaged fins, blisters, swollen tummy, cotton wool like growths, has there been any flicking/rubbing at all?
Sorry for tons of questions but your issue is a little vague.