What would happen to a human if he went to Neptune?

It would take about 12 years to get there. Assuming the human did not die on the long trip from starvation, boredom, dehydration or insanity, then s/he would certainly die upon arrival. If the high winds (over 1,000 miles per hour) didn't destroy the spaceship upon landing, and knock the sense out of the space traveler, Neptune is too cold and there is no oxygen.