A key-not-detected message concerns communication between the vehicle and its key or fob. Check that the correct key is present, then consult the manual for the designated backup starting method if the normal process does not work.
A weak fob battery is one possibility, not a complete diagnosis. Note whether a spare key behaves differently and whether the message occurs only in certain situations. Share those observations with 12th Street Auto Care Center; avoid assuming the vehicle will restart after it is switched off.