How to tell: The Layers dialog gives you ability to toggle the visibility of each layer on or off. Look at the Layers Dialog, and see if the layer you are trying to work on is active (i.e., darkened) and has an eye symbol to the left of it. If not, this is your problem.

How to fix: If your intended target layer is not active, click on it in the Layers dialog to activate it. If none of the layers are active, the active drawable might be a channel—you can look at the Channels dialog to see. This does not change the solution, though. If the eye symbol is not visible, click in the Layers dialog at the left edge to toggle it: this should make the layer visible. See the Help section for the Layers Dialog if you need more help.