Hard Reset variable change in MCU
if the microcontroller detects a hard reset, can the current
  activation password be saved in non-volatile memory before resetting
  and use it after reset?


Yes. The MCU won't detect the reset until after it has occurred, but provided the password was saved before the event causing the reset it will work.

However there is a chance that the saved password is corrupt (if there was a random power failure during saving or the MCU 'malfunctioned') so you should also write a checksum or CRC code to verify that the saved password is valid.