If your friend has a card reader, put your card in the card reader and delete all files and folders on the memory card (do not format the card with the computer). When deleting the files and folders do not let the deletions go to the recycle bin (hold down the Shift key when deleting).
Then put the card back in the camera and it will likely recognize the card.
Before you do anything else, format the card with the camera.
Failure to format a new card with the camera, can bite you a week later or even a month later. I did it twice before I learned my lesson.
Camera: Canon SD300
Memory: 1GB SanDisk SD card
I recently tried using my camera, but the display said that my memory card was locked. I removed the card and it wasn't locked. I inserted it again but I had the same problem. I locked/unlocked the card and tried again with no luck. I tried using a different card and didn't have a problem. I put the "bad" card in another camera and it read the same "Memory card locked." I tried connecting my camera to my computer through the USB cable it came with but no luck.
Before all of this happened I took some pics and my friend transferred them to a CD through his card reader. Since then, all of this happened. I'm going to try my friend's card reader to see if it'll recognize the card. Other than that, any ideas how to fix this? Anybody experience this?