Make sure the layer isn't locked by selecting it and looking at the padlock in the layers panel. If all else fails, dump your settings file by closing Photoshop. Then, hold down CTRL ALT SHIFT all at the same time while you start the program. It will ask you if you want to delete your settings file. You can tell it yes. This will not delete any of your styles or presets. It just resets things. If your using a MAC, it would be CMD ALT SHIFT.
“One must still have chaos within oneself, to give birth to a dancing star.”
― Friedrich Nietzsche