Possibly a few cells in the battery are dead. I have had the batteries in both the X5 and 335i replaced. They are both less than 4yr old and batteries on average last 5yrs, so it is an issue with BMWs and/or the batteries. If you leave the keys in the car not only the radio but all the electronics keep humming. If some of the cells are dead the battery will not provide enough current to start the car, the car may crank but there won't be enough current to spark. The dealer may be able to give you a loaner battery which may allow you to take the car to dealer for a checkup.

Dealer replaced the battery last year dec 2010, and the car has only 46,500 miles on it. Don't know if the battery goes bad that fast.

If the battery did go bad it wouldn't crank. If it cranks it isn't the battery

Would a faulty O2 sensor cause such an issue?

A faulty O2 sensor would throw a code

Not specific to O2 sensors, but just a comment.

There is a tendency for posters to want to skip the diagnosis and jump to "the answer". Two different problems with similar symptoms can have very different root causes and thus very different cures. A problem like "not starting" is a classic example of this. We need more information from the OP.

In the meantime, he said it cranked fine, and later added that it is a new battery. Yet most of this thread is about the battery.

Bingo.

(I missed the 'it still cranks fine' part, which was a critical bit of data.:()

Oh true... though when I had a burned wire that misread the fuel level, no light was on. They had to trace the wire. Perhaps it's something similar. But it seems this kind of thing should throw a code. Plugs would be my next thought, but again should throw a code.

"My car doesn't start, any ideas?" reminds me of the time my wife called me at work and said, literally, "My printer doesn't work, what's wrong with it?"

I was like, "Ummm, how would I know?" We were both in tech support at the time too. I still give her a hard time about that to this day.

Anyway for the OP - you're missing either fuel, air, or spark. Could be any of the myriad of systems providing those three elements to the engine.

Had my car towed to dealer this morning. Till late afternoon, SA called and said they still had not found out what was wrong with the car. He would call back around mid day tomorrow. Looks like something really serious, that they spent that must time still cannot find out????

SA called back and said the problem is "BAD GAS". I just filled up that night and the next day couldn't start. The total for clean up and change the gas pump is $4100.00. This is really expensive for just fill up the bad gas from a reputational "SHELL" station.