Solution:

SD and SDHC memory cards have a write-protect tab.
If the [Write Protect Tab] is in the locked position, the following screen appears.

During shooting :

If [Card locked!] appears in the upper portion of the screen, and [Recordable shots] or [Elapsed Time] shows [0]*, shooting is not possible.
* The above indications may not appear depending on the model of your camera.

During playback :

If you press the [Single Image Erase] button () to try to delete an image and [Card locked!] appears in the lower portion of the screen, the image cannot be deleted.

If [Card locked!] appears on the [LCD Monitor] of the camera, the [Write Protect Tab] of the memory card being used is in the locked (downside) position, so you cannot capture or delete images.
To unlock the tab, slide the [Write Protect Tab] of the memory card upward.