Answer: When I run across this type of situation, I go through a standard checklist to see if I'm stuck in some sort of mode that is preventing most of Photoshop for being available. Below are some of the things you can check for:

1) Is a tiny selection active making it so thing don't seem to be working when in reality a tiny area of the image is changing? Just click on the Select menu and choose Deselect if that option is available. If it's not available, then no selection was active.

2) Make sure you're not in the middle of a transformation (Edit>Transform) by pressing Return of enter twice to finish the transformation (twice just in case you have a number active in the Options Bar, the first one would accept the number while the second one completes the transformation.

3) Is QuickMask mode active. Glance at the bottom of Photoshop's tool palette and look at the two icons that appear below your foreground and background colors. If the icon on the right is active, then quickmask mode is active, so click on the left icon to get back to normal mode.

4) Check the mode of the image by choosing Image>Mode. Many features don't work on images that are in Indexed Color mode (convert to RGB in that case), also some features don't work on 16-bit images (so choose 8 Bits/Channel from the same menu).

5) Is a hidden layer active. Just check the layers palette and make sure the layer that is active is also visible (eyeball icon turned on).

6) Does the tool I'm attempting to use have any unusual settings specified? To reset the tool, Control-click (Mac), or Right-click (Win) on the tool icon that appears in the Options Bar that extends across the top of your screen and choose Reset Tool.

I'm sure there are more things you can check for, but those are the first ones that come to mind.