If you hold a stretched ideal (and therefore massless) spring with your hand (other end of the spring being fixed to a rigid support) and release it, the potential energy of the spring decreases but since sprjng is massless therefore increase in kinetic energy of the spring is also zero.So where does the potential energy go.
A mass less spring is a concept more than a real thing.
This concept is used to mathematically describe a situation in which some energy is stored, and can be released, to turn into a force acting on some mass. this mass can be some other mass attached to the spring, or the mass of the spring itself.
So mathematically speaking, if you had a mass less spring, it would gain infinite acceleration, but this is not physically possible.