I recently noticed a loud tapping sound coming from my exterior a/c unit. The a/c was working great but the noise was very loud so I opened the top to remove a piece of plastic that was being hit by the fan. After replacing the top the a/c blows, but is not cold. I didn't do anything but open it and close it and now its not working. The plastic didn't appear to be a major component, just something that lined the top of the unit. Any suggestions?