When you restore a LUN from a Snapshot copy, the LUN reverts to the state it was in when the Snapshot copy was made: the restore operation overwrites all data written to the LUN since the Snapshot copy was made.
A LUN restore recalls a selected Snapshot copy. During a restore, the entire LUN drive is restored from the Snapshot copy. For a restore to succeed, no open connections can exist between the host machine (or any other application) and the files in the LUN.