Your BMW might not start for several reasons. Generally, you can narrow it down to a fuel system problem, an ignition system problem or a mechanical engine problem. When you determine the system that has the problem, diagnosing the faulty component in the system will be a lot easier.
Remove the spark plugs from the engine. Consult your service manual for exact instructions on how to do this for your engine and year model. Clean and dry the spark plugs if they are wet with fuel, and reinstall them.
Remove a spark plug wire from a spark plug. Insert a small screwdriver into the end of the plug wire and hold it close (within an inch) to the engine. Have a helper try to start the engine. If a bright strong spark is not present during cranking the problem is most likely in the ignition system. Remove the coil wire from the ignition coil located on the passenger side fender well. Have the helper crank the engine over while you hold the coil wire close to the coil. A strong spark should be present. If the spark is present at the coil but not at the spark plug, replace the distributor cap and rotor located on the front of the engine. If no spark is present, replace the crankshaft position sensor located on the bottom front of the engine behind the crankshaft pulley.
Remove the air cleaner, and any associated ducting, to gain access to the throttle body. Spray a liberal amount of carburettor cleaner into the throttle body. Try to start the engine. If the engine starts momentarily and then stalls, the problem is in the fuel system.
Locate the fuel pump relay. It is located on the side of the under-hood fuse block, grouped with several other relays. The fuel pump relay is the one that is farthest away from the firewall. Unplug the relay from its electrical connector. The BMW uses a standard Bosche style relay. Two of the four wires attached to the connector will be large gauge wires that supply power to the fuel pump. The remaining two are smaller gauge and are the control circuit that turns on the relay. Connect the two large wires together with another piece of wire. Try to start the engine. If it starts and runs, use the circuit tester to verify if one of the remaining wires has power and the other remaining wire has ground. If this is the case, replace the relay. If not, replace the crankshaft position sensor. If, after connecting the two power wires together the engine still doesn't start, replace the fuel pump.
Remove the bolts that attach the timing cover to the front of the engine. Pull back the timing cover and have a helper crank the engine. If the timing belt is broken, the engine will spin over, but the timing belt will not move. If the timing belt doesn't turn the camshaft, the ignition system will not function either.
Another telltale sign of a broken timing belt is the sound the engine makes during cranking. When the belt is broken, a wiring sound is made by the engine instead of the rhythmic sound it normally makes. If the timing belt is found to be broken, make the necessary repairs and retest the systems. The engines in BMW cars are interference engines, and if the timing belt breaks, the valves in the cylinder head will be damaged.