2001 BMW 325 Car cranks over and over and will not start

This vehicle has been very reliable up until 2000 miles ago. We had air filter, all spark plugs and all oxygen depletion sensors replaced. Oil separator hose was replaced due to an oil and vacuum leak. Car ran perfectly after repair for 1 month although I ran it out of gas about 2 weeks ago(stupid I know). It ran perfectly again after I filled it with gas. I parked the car last Friday night and went to start it Monday morning. After 1 crank I thought it was going to start as usual but then it went to a higher pitched crank and did not start. The battery is very strong. If I continue to turn it over for 20 to 30 seconds the engine almost sounds like it will start but then does not and goes back to the same sounding crank. Seems like it may be getting a very small amount of gas after the 20-30 seconds of cranking but not nearly enough to run. Did I damage the fuel pump by running it out of gas? Could it be the timing belt? Any ideas? Thank you in advance for any input on this issue. Brian