I have had the exact situation occur with my '95 850, including the part about towing it to the dealer and it started without problem for the dealer. This first happened when the car was only a couple of years old. (How long have you owned the car?) I have never found the exact cause of my problem, but it does seem to occur when I have started the car and let it run for only a few seconds or a minute or so and then turned it off. After doing this, I can't get the car to start normally. I then learned that I could get the car to start if I continued to keep the starter engaged and let the engine crank for at least a minute without stopping. Don't press on the accelerator while you are doing this. I have had this occur at least 5 or 6 more times over the years and I have always been able to get it started using this method. Since I learned that it seemed to be related to having started the engine and turning it off again within a few seconds, I have stopped doing that. I have read some other posts that say this is like a flooding condition and you can prevent this by never letting the engine run for less than a couple of minutes anytime you start it. Now, if I am going to start the car up and just pull it in or out of the garage, I will let the engine run for at least 2 or 3 minutes before I turn it off again. Since I have been doing this I have not experienced this problem. Do you remember starting the car for a few seconds and then shutting it off just before this problem occurred?