Then hook it back up again after 10-15 seconds. I had this problem intermittently on my 528e, and we could never track it down because it happened so rarely. I found out after I had it towed to a shop and I took the battery out to use in another car in the meantime. When the shop hooked the battery up.... they called and said there was nothing wrong with the car. It happened about 5 times.. cranked like crazy, but no go. Some guesses about relays and resetting the computer, but nobody figured it out. A drunk driver solved the problem late one night by totalling the car as it sat parked on the street.