Why doesnt my adminlocker GUI work? When I input the password, nothing happens!

StarterGui>ScreenGui>AdminLock>TextBox>LocalScript

Any coding GUI related MUST be a Local Script.

local textbox = script.Parent

textbox:GetPropertyChangedSignal('Text'):Connect(function()
    if textbox.Text == "112Aa3C" then -- No brackets! '()'
        textbox.Parent.Visible = false
    end
end)

I created this simple script just for testing, but it is what you are looking for, and will solve your problem.

--Create a localscript, put this script in it, and put the script in your TextBox
local code = "InsertCodeHere" -- This is what you want the user to enter.
script.Parent.FocusLost:Connect(function (property)
    if script.Parent.Text == code then
        local part1 = Instance.new("Part",game.Workspace)
        part1.Anchored = true
    else
        script.Parent.Text = "Code not accepted."
        end
end)

Basically what this script does, is waits for the TextBox to lose focus (Pressing enter, clicking away, etc). Once focus is lost, it will determine weather or not TextBox.Text = code. If so, then create a part (or in your case, make invisible or whatever), if not, then it makes the TextBox say "Code not accepted." so players know it wasn't accepted.

The problem with your script is there was no way for it to even detect if the text value changed. so for you the script would be similar to this

local code = "InsertCodeHere" -- This is what you want the user to enter.
script.Parent.FocusLost:Connect(function (property)
    if script.Parent.Text == code then
script.Parent.Visible = false
    else
        script.Parent.Text = "Code not accepted."
        end
end)

EDIT: Seems incapaz beat me too it. His script with "textbox:GetPropertyChangedSignal('Text'):Connect(function()" would actually be better, as it will only detect if the text value was changed. Use his for a better solution.