Explain what happens to the energy that "disappears" from the water in an endothermic reaction
An endothermic reaction is one in which heat energy is added to the reactants in order for the reaction to occur. So the energy is transferred from the water to the reactants. If the water is being heated, it would cool slightly, requiring the addition of more heat to maintain the desired water temperature.
Some chemical reactions produce new stronger bonds. This means that potential energy is needed and this comes from, for example the kinetic (heat) energy of the reaction mixture. This means the mixture gets cold. No energy has "disappeared" only converted.