return duct

i have water in my return air duct. it is the first time. what can i do,and what is the problem

More info will be needed in order to give you a proper reply.
is this a main return duct or is it panning (joist lining)?
is there a water pipe within this return?
Is the duct near a humidifier?
how much water are we talking about?
Does the duct end at an outside wall cavity?

duct

I assume that this is an underground duct. You need a pump to remove the water. HOW the water got in there requires some investigation, because there are MANY ways it could have happened, not all of them serious.

Speaking of "returns", are there code restrictions as to what pipes are allowed to pass through an HVAC return?