Does it try to start and then die or just cranks over and over? If it just cranks, have you checked to see if you are getting fuel to the engine? Could be a bad fuel pump or clogged fuel filter. Pull a spark plug and crank it, if you're getting fuel then you'll get a squirt of fuel shooting out of the plug hole. Also check to see if you're getting spark. Pull one plug at a time and attach it to your coil or spark plug wire. Ground the plug while someone cranks the engine. If no spark at all from all 4 cylinders then could be a bad distribution cap/rotor. If it's distributorless ignition and no spark on all 4 then bad ECU. If it tries to start then dies check the MAF sensor as well. That should give you a few starting points. 3 things are needed for an engine to start; air, fuel and spark.