Magnetic force becomes zero as the particle is at rest in this frame of reference and we know that force on a particle,F=qvBsin(θ′′>θθ) , where v is the velocity of the particle.So, when v=0,F=0 Magnetic field on the particles still exists because the current is independent of the frame of reference. For any reason, if the electrons of the wire seem to be at rest in a frame of reference, the protons are still flowing opposite to the frame of reference. Due to this, the current and the magnetic fields still exist.