Some people have had problems with the SD card that re-seating it (popping it out and back in again) have fixed. Others had to get new cards from the dealer. You might want to try re-seating it and see if it works.
If it doesn't then try another SD card with some music files and see if it can read that card. If it does then it's probably the SD card and not a problem with the reader itself. If it can't read the music files, then it might be a bad SD card reader.
Basically, with either a bad SD card or a bad reader you would have to contact your dealer to get fixed. If it's just the SD card, it's a much easier fix that won't mean you lose your MKX while it's in the shop.
This is probably more what you were looking for when you originally posted. Hope it helps.
-Tonya