A stretched band can return to its normal state if it is allowed to do so. The energy stored in a stretched band is in the form of potential energy which arises due to the position of the particles that make up the band. This gets converted to kinetic energy when the band is no longer being stretched.
Kinetic energy is energy in a moving particle and is given by the formula KE = (1/2) m*v^2, with m being the mass of the particle and v being the velocity of the particle.
A body can have a potential energy stored in it due to many causes some of which include potential energy due to the gravitational force, the electromagnetic force and many others.
Potential energy is converted to kinetic energy if the body is given an opportunity. Like if the stretched band is released, all the particles that make up the band accelerate and return to a neutral position where they have zero potential energy.