Hi, I Purchased a 1999 BMW 528I and all was running well until the other day...

With a 15 yr old vehicle it can be just about anything. No spark condition at ignition coil, no fuel pressure at engine, unmeatered air intake leak etc... I would let them do their due dilagence and troubleshoot the issue without any input. I see too many instances today that regular people go to the internet and read a post/suggestion and strongly believe that their vehicle is suffering from the same condition without ever testing a single component. Then when it arrives to a qualified service station. Vehicle has a couple of parts installed, frustrated vehicle owner, yet still full of suggestion/ input. Just beware must automotive technicians have hundreds if not thousands of hours of training/ experience at their skillful craft. My only ews suggestion is if you have another authorized key try it to see if it starts w/ that one. Best of wishes.