Also, if by "doesn't take" you mean that you get a black screen for an image, then the camera may have been turned off before it had a chance to write the image to the media card.

Good point, Deb! And you may not be able to realize it if you use an optical viewfinder.
Some digital cameras can be timed to automatically blank out the screen to save battery drain. Check your manual as manufacturers vary in what they call the feature (eg. Monitor Options or Power Save Options).
On mine, when the LCD goes blank and I press the shutter-release button, the screen goes back on but no photo is taken. When the LCD is in power-saving mode, depress your shutter-button down half-way to reactivate it. When it's back on, then proceed to take your shot.