What holds up a sail on a boat?

A mast.

Although it is attached to a mast the thing that holds a sail up is a halyard.

a mast is something that pushes the boat with wind and the reason its called a mast is cause it holds the biggest sail, the thing your looking for is something that is on the bottom of the boat and runs vertical on the boat. if that gets ruined the boat is ruined, unrepairable