how do i return the camera to the player properly?

Hi, assuming your using a localscript to change the players camera. This script should reset the camera.

EDIT2: [This should fix most issues]

local target = workspace.Part 
local camera = workspace.CurrentCamera 
camera.CameraType = Enum.CameraType.Scriptable 
camera.CameraSubject = target 
local angle = 0 
while wait() do 
    camera.CoordinateFrame = 
    CFrame.new(target.Position) --Start at the position of the part 
    * CFrame.Angles(0, angle, 0) --Rotate by the
end

That is your current script and at it's state it just continuously rotates the camera, so I believe that you can use a global variable to solve this

(This is the gui script)

-- once again used in a localscript

local button = script.Parent -- change this to where your gui button is
local cam = game.Workspace.CurrentCamera
shared.camRotate = false

button.MouseButton1Down:connect(function()
    cam.CameraSubject =  game.Players.LocalPlayer.Character.Humanoid
    cam.CameraType = "Custom"
    shared.camRotate = true
end)

(This is in your cam rotate script)

local target = workspace.Part 
local camera = workspace.CurrentCamera 
camera.CameraType = Enum.CameraType.Scriptable 
camera.CameraSubject = target 
local angle = 0 

while shared.camRotate == false do 
    wait()
    camera.CoordinateFrame = 
    CFrame.new(target.Position) --Start at the position of the part 
    * CFrame.Angles(0, angle, 0) --Rotate by the
end

Hopefully that fixes the issue