The nomination is over if this happens.
Why would that be? Frankly I don't get how Wisconsin is do or die for anyone. It only has 42 delegates, and it's likely that at least one CD goes to the statewide loser anyway. You could argue that the winner will pick up momentum, but there basically hasn't been any momentum in this contest from day 1, only demographics and the Cruz-is-awesome-at-caucuses effect.
The GOP-e has pulled every trick in the book out to deny Trump a win in WI. If he somehow defies the polling and wins the state, they were once again rejected by the voters.
The narrative will be that he's the presumptive nominee. He will go on and sweep NY on 4/19 and the NE Super Tuesday Primaries on 4/26. Cruz will have no path to the nomination.