i replaced eveythin in this car..and now it wont start

It all started with the radiator..I replaced it..and it was over heating..I replaced it again..and it kept over heating..so I replaced the following..thermostat,water pump,all the hoses that's has to do with the coolant going thru..,the catalyst converter..battery,radiator fan,the gaskets are good..so it's not the head's..umm radiator cap..I mean I put bout 1000$ this past week..before I took it to my mechanic it would turn on..now after he put all them parts in me it won't start Smh..so he checked the computer reset it and nothing..now I'm going to take it in pay another 100$ for a diagnostic because he's tester can't read it's code and pay more after that..I invested too much in the car to give up..so the overheating problem is fixed..he drove it for a bit ..brought it back to his shop to replace the rear springs..and after he was done he tried to started it won't start..all the lights turn on he checked the alternator and it's good too...we think it could be the computer,a fuse,or ignition or the car not reading the key..I don't Kno..but I just can't give up