How do you find out why a GFI won't reset if it tripped several times due to Christmas lights and rain but they are now unplugged and when the reset button is pushed it immediately trips?

If the outlet cover was held open by the lights it could be wet. A smal amount of water will cause a GFI to trip. A GFI is a device which makes certain that voltage is not draining away from the two wires, to some OTHER voltage drain, such as a human body or a wet circuit. if above not the answer go to your breaker box and find the breaker that controls that circuit. you will know when you have the right one when the breaker does nothing, not even trip it self. see what else is on that circuit {what else turns off when main breaker is tripped and unplug everything that is on that circuit then turn on the main breaker. try the reset with everything else off if it still trips then its the gfi itself. they go bad often on construction sites [my own personal experience] if it resets properly than one of the other things on that circuit is the culprit. replug or turn them on one at a time. the breaker will trip if its one of these. if not replace the breaker. pretty cheap at lowes or home depot. turn off main breaker when you replace it. lots of luck!!