Refractory period of a myelinated nerve is due to:

Only when the membrane has repolarized up to about - 40 mV to - 50 mV that the Na+ channels recover from their inactivated state and go into resting state. All this while, the membrane is refractory; that is, the membrane can excite again only when many of the Na+ channels have recovered (which itself depends on the membrane potential). Hence, refractory period is due to the voltage-dependance of the Na+ channels.