Is the battery flat? Just because the radio works (1-2amps) your battery might not have enough power to start your engine (over 100amps). Turn your headlights on are they nice and bright, do they dim very low when trying to start, if so the battery could be low on charge or faulty, a fully charged battery should be 12.5 volts when fully charged.
Many people blame immobilizers etc. for a battery fault, buzzing noises, relays clicking, dash lights flickering, starter motors sounding like a machine gun are all normally battery problems.
If in doubt it is best to try jump starting or swap the battery for a known good one.
If YES read on if NO click hear
If so you will need to check the cam belt.
Are all the engine management and immobilizers warning lights behaving as normal? if any are flashing it normally means there is a problem with the immobilizer (try different key) check all fuses, immobilizers can take power from any fuse not always marked, Try leaving ignition turned on 5 minutes, some immobilizers will resynchronize there self, has the key fallen apart? It may have lost the microchip (this happens a lot with Vauxhall keys
Has the vehicle been started and turned off without warning up? (Moved car a short distance, on/off drive etc.) If so are the outside temperatures low? Many cars will flood if not warmed up in cold weather, try starting with accelerator fully depressed, releasing very slightly when trying to start, is the car trying to fire? Can you smell petrol from the exhaust? If so try one or two times as described, if no luck, either leave for a couple of hours and retry (as described) or remove the fuel injection pump fuse(see Micra problems), try as described, car should start and burn up excessive fuel and cutout, replace fuse and retry. (Note when car starts don't release accelerator straight away hold about 2,000 rpm for 1-2 minutes or car may re-flood, gentle releasing to idle speed) Fully warm car up before switching off.
If still non start, but you can smell petrol the next stage, is to remove the spark plugs (they should be wet with fuel) if possible first clean un-burnt fuel off with carburettor cleaner and then warm the electrode up with a blow torch (you can use the grill in the kitchen but don’t burn your fingers) if none of the above are available just clean electrodes as well as possible with dry cloth. Refit and try as described.
Can you hear the fuel injection pump run (Buzzing for a few seconds when turning on the ignition and continually when engine turning) most cars have a fuel injection pump in the tank, if in quite area you should be able to hear it running, (remove fuel filler cap and listen there if not sure or you may be able to tip rear seat and put you hand on top of pump). If you think the fuel injection pump is not running first check to see if the vehicle is fitted with an inertia fuel shut off switch.
Check the fuel injection pump fuse (best to check all fuses, some are not always marked correctly) is not blown, if ok, try banging on the bottom of tank with somebody else trying to start the car (Mondeo's pump stick when worn particularly).
If still not sure you can check for fuel pressure at the engine but this must be done with great care as unleaded petrol is very flammable, only test with cold engine don't smoke and have cloth ready to catch fuel. Many injections systems have a pressure test valve (like a tyre valve) on fuel rail gently press pin in middle you should have injection pressure if no valve try taking a fuel hose off of fuel rail.
If no fuel pressure it is not always the pump, fuses, relay's, inertia switches and wiring can all stop the pump running, we now really need to find the wiring to the pump and check we have a good live and negative to the pump when cranking.
(Note Rover 75 fuel pumps fall to pieces in the tank so the pump just pumps the fuel back into the tank and not up to engine). It is always worth adding some fuel to any car if not over a 1/4 of a tank especially if parked on a slope.
If happy fuel pump running next listen to hear if injectors are clicking when cranking (if possible use a noid light to check electrical pulse see Picture)
If they are, we now know the computer ECU is getting signal from crank sensor, so it must be working ok. If no pulse at injectors the next step would be to start looking a bit deeper, if possible check for a spark at the spark plug. This use to be easy but with modern coil packs can be difficult, best to use a proper spark tester as the voltages are getting very high in modern ignition systems and can give you quite an electrical shock.
If no spark or injector pulse the next step would be to check the Crankshaft Position Sensor (CPS) these can give non start problem particularly Fiesta/Escort (corrosion in connector) and Clio/Megane's (dirty sensor).
These are normally on the clutch bell housing, but can also be on the crankshaft pulley (next to alternator drive belts). If possible pull connector apart and clean connection with WD40(these sensors produce their electricity about 0.2-3 AC volts when engine turning, so can easily be lost due to corrosion) if still non start next check if sensor is working by using a volt meter.
Set meter to AC volts, you might need to attach some leads to sensor to test, loud speaker leads are often the right size for this, with meter set on Ohm's check the resistance should be around 200-500 Ohm's normally, if really high resistance try to find correct specification but normally would be faulty, if seems ok set meter to AC volts and crank engine being careful of moving parts, you should get a reading of between 0.2 and 3.0 volts with engine turning, if no reading remove sensor and clean, sensor some times are contaminated with particles from the clutch, which can stop them working.
Refit and test to see if producing AC electricity. Sometimes it can be very hard to get the senor out, a trick that will sometimes work is, if you can see a hole in the bell housing near the Crankshaft Position Sensor (CPS) sensor try spraying carburettor cleaner onto the flywheel, when cranking the engine, this will sometimes clean the sensor and the vehicle will start.
See video on testing crankshaft sensors.
This is quite hard to explain, please let me know if you can understand what I am trying to say, if not I will try to re-write parts that are not clear. I will go into more depth next year thanks Tony.