How Do I Clone It Back?

If you don't have a copy of the block as backup somewhere, destroying it would leave you with no way to get it back. Here's what I would recommend:

--Alongside the variable for "brick", also define a backup copy of that brick.
local brick = workspace.Part --I would really recommend that you use a unique name for the part you want to manipulate with this script, and change this line to reflect it, if you haven't already.
local brickBackup = brick:clone()

--When the brick gets destroyed
brick:Destroy()
wait(5)
brick = brickBackup:clone() --Make a new brick from the saved backup
brick.Parent = game.Workspace --Put the brick into the game
script.Parent.Text = "20"

Woops! This Code. Missed a bit.

local ten = 10
local nine = 9
local eight = 8
local seven = 7
local six = 6
local five =  5
local four = 4
local three = 3
local two = 2
local one = 1
local zero = "Round Over"   
local twenty = "Begin"
local brick = workspace.Part



 for i = ten, 0, -1 do
        script.Parent.Text = ten
        wait (1)
        script.Parent.Text = nine
        wait(1)
        script.Parent.Text = eight
        wait(1)
        script.Parent.Text = seven
        wait(1)
        script.Parent.Text = six
        wait(1)
        script.Parent.Text = five
        wait(1)
        script.Parent.Text = four
        wait(1)
        script.Parent.Text = three
        wait(1)
        script.Parent.Text = two
        wait(1)
        script.Parent.Text = one
        wait(1)
        script.Parent.Text = zero
        brick:destroy()
        wait(5)
        script.Parent.Text = twenty
        wait(3)



   end