Common causes are bad starter (relatively high failure rate), key sync issue, ignition switch, or SMG pump. You seem to have a 6MT so skip the last one, but I would first try another key, then troubleshoot the ignition switch, and then move on to the starter.
I suggest you also edit your top post to include more detail and information about events leading up to the problem.
540i - Stock/Sport / 6MT / 13.9@100mph
WS6 - 231/234 / 6MT / 4.10 / 410rwhp
ITR - 262/262 / 5MT / 12.2:1 / 9600rpm
540i/6 Sport, Jet Black, Style 65, LED tail, DICE+GROM USB, !CDV, Redline MT-90, M1 0-40w