Just completing your phase doesn’t mean that the hand is over, you would also have to actually go out. Your opponents have until someone goes out to also complete their phase.
If you meant that you did go out, then the hand would end immediately. You move to phase 2 while your opponents stay in phase 2. They count up the points in their hands and add them to the their score. Then a new hand begins; no cards are kept from one hand to the next.
It makes no difference if the player who goes out is a phase behind the other players or not. All players have until someone goes out to make their phase.
If you successfully make a Phase, then you try to make the next Phase in the next hand. If you fail to make a Phase, you must try to make the same Phase again in the next hand. As a result, players may not all be working on the same Phase in the same hand.
After laying down a Phase, players try to “go out” as soon as possible. To go out, you must get rid of all of your cards, by discarding or hitting on an existing Phase. The player to go out first wins the hand. The winner of the hand, and any other players who also complete their Phase, will advance to the next Phase for the next hand. Players total the cards left in their hands. (The fewer cards left in your hand, the better!) All the cards are then shuffled and a new hand begins. (Remember, if you did not complete the Phase before another player went out, you must work on the same Phase again in the next hand.)