The first thing the customer should do is use the troubleshooting guide in the manual. If nothing in the manual helps.

And check the dehumidifier has power to it.

 The customer should unplug the dehumidifier and take off the filter and gently clean it with soap and water. While the filter is off, look inside the back of the dehumidifier and look for a white piece of plastic about the size of a matchbox. It will have lots of little holes in it. Once they have found it, get a hairdryer and on a cool setting blow air all around it. Once the filter is dry, replace it, plug the dehumidifier back in and try running it. 

 If there is no power or if the warning light is still on they will need to contact the customer service dept. of the company they purchased it from