The Republican and the Democrat have a tie in the Electoral College 269/269.
All the electors remain faithful and cast their electoral vote as their state voted.
The election of course goes to the House.
The House votes 25 state delegations for the Republican and 25 state delegations for the Democrat.
How is the tie in the House broken or what is the next step?
The precedent is well established. Until and unless 26 States vote in favor of one or the other (or possibly one of the three if a faithless elector voted for someone else to be President than he was supposed to) there is no President.
Over in the Senate, they'll need 51 Senators to chose who will be Vice-President and thus Acting President until the House chooses a President. And I do mean 51 Senators, not 50 plus a VP. The VP does not get to break this tie, and even if not all 100 Senators show up to vote, it still takes 51 to elect.
If neither a President or a Vice-President is chosen then under current succession law, whoever is Speaker on noon, January 20, becomes Acting President until one of the two elections is settled. (Assuming the Speaker is eligible, if not then it continues down the list until someone eligible is found.)