Most filters apply only to RGB images. Check the status of Image: Image > Mode and change it to RGB if needed. A few filters have other requirements. For example Image: Filters > Enhance > NL filter is grayed out if the current layer has an Alpha channel.
If you’re trying to edit an image and your actions seem to have no effect, check the following points:
Is there a floating selection? If so, you need to either delete the floating selection or place it into a layer before doing anything else.
Is the layer you want to change the active layer? The active layer is emphasized in the Layers dialog.
A selection may not be visible either because it is too small or because it ...