my e39 does this if I start it first thing in morning and switch it off a few seconds later.it will not start no matter how many times its turned over..the only way I found was to take the plugs out and dry them off and leave the plugs out for a few hours,stick the plugs back in and fires first time.it happened me 3 times in 2 years and is a real pain if you forget..done some research and seems to happen to a few bmw engines,mines the m52 but apparently the m54 as well..