My Canon powershot SX240 hs has corrupted an old SD card?

I wanted to see how the files of an old 256MB SD looked on my new bright, crisp screen powershot. Evertyhing looked fine but then a memory card error message appeared, so removed it an replaced in the old Nikon that had taken the shots - now getting message memory card not formatted message on the Nikon.
Anyone any ideas what might have happened and what are the chances of me recovering the files?

To make matters worse my laptop is being repaired at the moment, so I can't do much research on line.
Would the camera shot be able to help?

Canon has a help line which is free.

You old memory card is tiny.

Maybe you need to buy a second or third SD card that is at least 1 gb or better 2 gb.

Once you format it using your P&S camera, you will be able to continue shooting what you like

Once you notebook is returned, repaired, you can download Transcends program, RecoveRx and recover the corrupted images on your old memory card. Once you have done that, format the old card using the format feature either on your camera or the one provided by RecoveRx.

From this point on, at the end of each shooting day, copy all the images to your computer and once they are safely there, format the card using the format feature on your camera.

If you have two cameras, make sure that you use a different memory card for each one.