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When suddenly nothing works anymore
There are several situations where you may wonder why some tools do not work. Here are some things to check:
  • Many tools (for instance, the paint tools) only apply to the "selection" and if you have a selection the effect of these tools is restricted to the selected pixels. In some cases a very small selection is created by mistake, so painting in most parts of the image does nothing. This is easy to check: in the "Select" menu, "Select/All" and Select/None" are both available when there is a selection on part of the image. Click "Select/None" to remove the unwanted selection.
  • Tools apply to the selected layer, so check in the Layers list (Ctrl-L) that the right layer is selected (blue highlight). Tools also have no effect outside of the selected layer boundaries if this layer is smaller than the canvas (you can enlarge it with "Layer/Layer to image size")
  • A corollary to the two items above, if the selection and the active layer have no common pixels, nothings happens.
  • Locked pixels cannot be altered. Check the pixel-lock and alpha-lock status of your layer at the top of the Layer list.
  • Many filters only work on RGB images, and the corresponding menus will be disabled/grayed if you have a grayscale or indexed image. The type of the image is next to its name in the title bar, and can also be checked in the "Image/Mode" menu.
  • The tools in the "Colors" menu do not work on indexed images (GIF images are color-indexed), but nothing makes that obvious, the tools just seem broken in that case. 
  • If you have a "floating selection" ("Edit/paste", or after using transform tools with a selection active) many operations are forbidden until this floating selection is either "anchored" or transformed into a plain layer. The floating selection is visible in the Layers list.
  • Paint tools may be put in a "paint mode" that has little effect on the pixels you are using. Check that that the paint mode is "Normal" unless you really want it otherwise.
  • Check the brush you are using. If set to the 'clipboard', the clipboard contents may have changed. Try with another brush.
  • Check the brush opacity
  • Check the layer opacity
Thank you so much Ofnuts. Very useful tips to have in mind.
Something I found only recently too is:
When painting with a mask. the layer has to be visible.

(A couple of times I wondered why no colour was showing through and I forgot I had turned the layer eye off)

Great list Ofnuts.

Another place to check is tool options. I nearly tore my hair out recently when the Move tool didn't move anything. Then I discovered that it was set to move selections and I had no selections. Tools like Scale, Transform, and some others are also specific to layers, selections or paths.

Fortunately, the GIMP error console is great at pointing out errors like these. I keep it loaded in my dialog docker.

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