Since so many operations rely on layers, checking the Layers palette should be first on your checklist. You need to ask yourself:
Am I on the correct layer?
Is the target layer hidden?
Is the layer active?
Is the layer locked?
Should the layer be locked?
Is the layer part of a clipping mask?
Is the correct thumbnail active (layer versus mask)?
Is more than one layer selected by mistake?
Should I have multiple layers selected?
Are layers linked that shouldn't be?
Am I clicking on a Group rather than a layer?
Are the layers stacked in the correct order?
Am I attempting to work on a Smart Object rather than an image layer?
Are the Opacity and Fill percentages appropriate?
Is the layer blend mode set correctly?
If I'm adding a new layer, have I clicked in the appropriate part of the layer stack?
Phew, a lot of stuff to check, right? And that's just the Layers palette! But it's the answers to questions like these that cover some of the most common roadblocks people face. Okay, you've checked all of the Layers issues, so what's next ?