Check Out the Layer

Confirm that the layer you are working on is not hidden by checking the layer panel. To make the layer visible, locate the eye icon on the left side of the layer panel, and if the box is empty, click on it to make the layer visible. If the stamp tool stops working, select the layer you want to clone and press Alt/Option to bring it back to life. Additionally, make sure to use the correct layer; the adjustments will be sampling the wrong parts if the wrong layer is chosen. If this doesn't solve the issue, your recently updated photoshop version may be the cause; in this case, try reverting to the previous software version.