I will add 1 more less likely guess - the immobilizer.
Try the other key in case that one has suddenly lost its chip coding.
But could also be the system (EWS) that reads the key and communicates with the computer to signal that the starter (and fuel pump, etc.) can be energized.
However, starter motor is the prime suspect - failures in later model X3's are not unheard of.
These are the 3 things that will prevent a jump start from working, although for the battery to be the culprit it would have to have suddenly developed a complete internal breakdown.