The good news is your bridal wreath (Spiraea) is one tough shrub and new growth will quickly sprout from the roots if cut to the ground. The branches that were cut will not very likely root due to the woody structure and time of year. You can try to take pencil thick cutting from a cut branch that is not brown and woody and not green and supple but semi-hard. Remove the lowest leaves from the bottom half of the cutting and poke it into moist soil. Keep the cutting misted and shaded. Roots may form in a few months. By that time your original shrub will have sprouted fresh new growth and I predict you will be happy with the shape and vigor of your newly but accidently pruned Bridal Wreath spiraea. Keep growing, Marianne Binetti