You don't toast it _all the way_. Heat it up a modest amount, just enough for the ice crystals to melt, and it will spring to life as if it were a practically fresh slice of bread (until it is freezer burnt, at which point it will be a little bad).
The method I was taught was to wrap the slice of bread in a paper towel and microwave for 5-10 seconds.
The method I was taught was to wrap the slice of bread in a paper towel and microwave for 5-10 seconds.