In general, there are two main issues (and a possibility) when this occurs:
Bad SD card. (I use fat32 on CGo3 cards, many other folks use different formats)
Bad Wifi board. (Card reader failure is fairly common)
Far less often, a camera firmware or update glitch.
Have you tried different cards?
Anything related happen between last time it worked and first time it did not work?
Was this the Q500 in the farmer's field for a couple months?