Question
I have 20 Leyland Cypress Trees we planted 9 years ago to provide privacy from a house that looks down on us.  We trim them every year so they do not grow higher than 8-10 feet.  

Our neighbor reached 2-3 feet over our fence on to our property and cut the branches at the trunk level.  You can now look through the trees and we have lost a good deal our privacy.

My questions are...

Will the branches re-established themselves after they have been cut to the trunk thus restoring our privacy?  

Will major pruning of the branches harm the trees?  

Do we need to replant new trees to restore our privacy?

Thank you for you help!

Answer
No the Leyland cypress will not sprout new limbs when the limb is pruned to the trunk. The pruning does not harm the trees health but as you say will reduce the thickness of the foliage. Before I replaced them I would try fertilizing the tree to see if the fertilizer will add growth to the foliage on the side not pruned and give you the effect you want. This fertilizer will contain higher amount of nitrogen, phosphorus, and potassium and always buy fertilizers with trace minerals, also called minors. (example: 18-8-8 plus minors). one of these type fertilizers is called Espoma Hollytone. Use directions on the bag and apply about 1/2 the recommended amount. Especially in sandy soils, Leyland cypress has shown signs of over fertilizing, and therefore it is better to split the application. Make sure you water the fertilizer in.   
The extra nitrogen will add the foliage. Check with your local nursery for this type of fertilizer.