Assuming it is not equipped with a digital display, which mode of the regeneration cycle is indicated while it remains unpowered?
Because most softeners require electrical power to advance the control valve from cycle to cycle, removing power mid-regeneration cycle, will often result in a constant flow of water to drain. As such, the water that is within the resin tank will likely remain fresh, so just plugging it back in is probably OK. If water was flowing through the softening resin media, without regeneration, the resin's capacity will be fully exhausted, so at least 2 regeneration cycles performed back to back will be needed before the water to fixtures will be fully soft, assuming the resin was not damaged.
If the softener water supply was bypassed so the water within the media tank is 1+ years old, before opening the outlet valve to supply soft water to the home, suggest first adding a few tablespoons of unscented chlorine bleach into the brine tank to sanitize the brine tank, then perform a full manual regeneration to draw the chlorine/brine solution from the brine tank into the media tank to sanitize the softener internals.
Once the sanitizing cycle has completed, wait 1-1.5 hrs to allow sufficient time for additional salt to desolve before performing a 2nd manual regeneration cycle. Once the 2nd cycle has concluded, restore soft water flow to the home.
What brand of softener?
What age of softener?
What is the water source (municipal = chlorinated) or private well?