I had a similar experience with my '93 530i. One morning it just wouldn't start at all. As in your case, there was nothing evidently wrong with the car. I called in the dutch equivalent of the AAA, and they couldn't find anything either. I had to tow it to the dealer's. Next day, it turned out that they initially couldn't start it either, then changed the battery, and that cured it. They said that there had probably been a short in the battery, so that the voltage remained too low, even with jumper cables from a second car as I had tried, causing the computer to decide against starting. According to them, they had had the exact same problem earlier that week, so apparently it is not that uncommon. So try sticking in a fully loaded brand new battery, and see what happens.