Water damage is weird. It can be several days after exposure to water before the device breaks. Right now you have 2 options.
1) Take it back to the store and hope that they will replace it under warranty, even though the device is water damaged. There are moisture sensors on the battery and inside the housing of the phone that have probably changed color from white to red. If they check these, they will know that the phone has been exposed to moisture, and they will probably not replace your phone.
2) Try to fix the phone yourself. Find videos on YouTube that show you how to disassemble the Atrix. Disassemble the phone until you can remove the main board (motherboard) from the phone. Soak the main board in 91% or higher isopropyl alcohol for about 2-4 hours. While it is submerged, use a toothbrush to clean the main board of any corrosion you can see on the metal contact points of the main board. Remove the main board from the alcohol bath and let it dry for a couple of hours. Reassemble the phone, turn it on, and pray.