As per standard in these situation - you need to check on 3 most important things that engine needs to run successfully: fuel, spark and air.
Firstly, I'd remove all the spark plugs, then you could see if they were flooded or if they are dirty. Remove fuel pump relay and crank the engine to see if you are getting spark. If that's ok, check your air intake box, rubber elbows and all the vacuum hoses you can reach/see. Make sure they are fine, well connected and with no cracks.
Then you need to see if you are getting fuel. You can check that on fuel rail. On some cars there are like a little valve that you can press on and it squirts some fuel out telling you there is fuel in the fuel rail. In cars that don't have that, you can remove the fuel line, the closest one to a fuel rail and check for fuel. You should also listen to a fuel pump go "click" once you turn your key into ignition position 2.
If all these work out fine, then it means that one of the sensors is messing up and this can be checked by diagnostics read.

I've been through similar situations and there many causes that could be a problem. Once the rubber elbow had a crack, engine was barely running and was very rough. The other time the vacuum hose from brake booster came off, the car didn't even start. Third time my crank sensor was counting last minutes, the car would sometimes start, sometimes not.