Hi gumuse, here are some things to check, first is new battery charged to full power?(charge battery or use jumper cables) Next are all the wiring harnesses clean and connections making good contact with their locations. Have you checked the fuse in the wiring harness,(bad fuse) Is there power getting to the starter solenoid?(bad starter), And last can you turn the engine over manually(engine issue)
Hope this helps you resolve the problem, if you need more help re-post with added details and the model number off the tractor.