There are several possible causes for "memory card locked", "memory card error", or "no memory card". Some include trying to use of a 4GB SDHC card with an older camera made before SDHC format came out. Others include problems with the tiny slide switch on the side of the card, or the electrical contact switch internal to the camera. The problem may also arise from simple formatting problems if the card has never been formatted in the camera (note that this will also erase the card). Troubleshooting tips for this problem may be found at this link.