You're falling into a common problem here. "Not starting" can mean a lot of different things, so you have to be specific as to what is happening. Does the starter crank the engine fast enough? Is there no sound at all from the starter?
A failed crankshaft position sensor will cause normal cranking but the engine won't start or run. You have to list which engine you have and the mileage. The crank sensor is usually pretty easy to replace, ... Takes less than an hour. The timing belt is a major job that requires you to leave the car. They are doing you a disservice if they disregard recommending a new belt because some engines are "interference" engines and will have a lot of expensive damage if that belt breaks. They should recommend replacing the belt before that happens, but it's usually based on mileage. Replacing a sensor is not the time to replace the timing belt unless it's called for.
Every manufacturer has a mileage recommendation that you will find in the owner's manual. It's a good idea to not wait until that mileage occurs. If the interval is 100,000 miles, which is rather high, have the timing belt replaced by around 80 - 90,000 miles to be safe, but not simply because some other repairs are being done.
Monday, January 4th, 2016 AT 6:05 PM