Locked media card??

I uploaded my photos from my media card yesterday, put it back in the camera and didnt use it again. Just now went to take some snaps of the dogs being horrors and it says that the cards 'protected'. I dont have a clue how I have done this. I got an old card an put it in (incase it was the camera playing up!) and it worked fine, I also put my card in my old carmera and that says the card has been locked?

How have I done this, and how do I unlock it

Thanks.

On an SD card there is a little switch on the side that you can slide up or down.

It happened to me once and I missed so many photo opportunities

Opps that simple I feel a right wolly now

Thanks alot