A particle with mass $m$ moving with constant velocity $v$ has a total energy $T$ equal to its kinetic energy. If we consider the reference system (inertial) moving with the particle, the velocity is 0, so the total energy is 0. Where is the energy gone? What about energy conservation law?
On selecting the frame of reference of the moving body, then all other objects(at rest with respect to previous frame of reference) are moving with velocity 'v' relative to the body and hence acquire kinetic energy. The energy which is possessed by an object in one frame of reference can be the energy possessed by other objects in another frame of reference.