Canon powershot A540 digital ...problem it shown card locked

It's a lost cause. The A540 works with SD cards. While SDHC cards will fit into the slot, they are electronically incompatible and will not work in devices not designed for them. You need to stay with SD cards, 2GB and smaller.

There are several possible causes for "memory card locked", "memory card error", or "no memory card". The most common is trying to use a 4GB or higher SDHC card with an older camera made before SDHC format came out. Other causes 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.
You also mention recovering photos saved on the card. If you actually did take pictures with this card (assuming it's a 2GB or less standard SD card, as 4GB SDHC was not designed for your camera), you may be able to recover the photos using recovery software. See this other link.

First of all, sorry that your post was overlooked for so long.
For the card locked problem, see this.
For recovering your photos, see this.

Have you noticed the slider on the side of the card? This acts as a lock and might have been accidentally moved to the locked position, in which case you could just move it back to unlock again. Hope this helps.

Look at your memory card. SD cards have a slide switch along one edge. The position farthest from the contacts locks the card, protecting it from writes. The position nearest the contacts unlocks the card.

When this problem occur you need to set the memory card unlock or not protected. Remove the memory card from camera and you can see a small button in the memory card, so you need to slide that to set the memory card unlock or not protected.

The A540 works with SD cards. Your 4GB card is actually an SDHC card. While it fits into the slot, it is electronically incompatible and will not work in this camera. You need to use SD cards, 2GB and smaller.

Hi there,

Here is how you do it.
1) press the menu button.
2) scroll right to the Setup Menu
3) scroll down to format.
4) scroll right to enter Format Menu
5) scroll up to Low Level Format.
6) scroll left or right to add a check mark.
7) scroll down to ok and press the function/set button.

Sorry no one got to you sooner. Hope this helps you, and thanks for coming to FixYa. Don't forget to rate my solution.

Rob

There's a little switch on the side of the SD card. Remove it from the camera and find the switch. Normally, there's some very small printing next to it saying something like (Lock / Unlock).

To view the pictures on the computer, remove the card from the battery compartment and put it in a card reader connected to your computer.