Jus changed thermostat and radiator on my 04 kia rio now wont start...?

Does the car try to crank or nothing happens when you turn the key. If nothing happens recheck your wires and connection to battery. You did disconnect your battery when you did all the work. If you did not check fuses. with the battery hucked up you may have struck a arc and blown a fuse. I am really guessing because I don't know what you car is doing when you try to crank it.
I changed the radiator in 04 rio a few months ago and I did not have any trouble.
Have Codes read if possible that usually give me a starting point.

Did you bleed all the air out of the cooling system after you put all the coolant in?
Some cars may not start, because the computer thinks there's no coolant in the engine.
Hope this helps.