San Disk card locked?

I took some pictures on my digital camera then removed my sandisk card and inserted it into my computer to transfer the pictures.  When I put it back into my camera I get a message that reads “card locked” and it prevents my camera from working.  How do I unlock the card to resume use?

I just realized there is a lock “switch” on the side of the card - I have unlocked it and it is working again.

Thank you soooo much for this post.  I have had this card for 5 years and it had never been locked 'til this past summer.

I should have researched and found your post eariler.  

Just last week I gave up on trying to figure out the camera settings that “caused” (or so I thought) the error and bought a 4 GB Sandish Ultra 15MB/S  SDHC and now it too has the same problem.  However moving the switch on the card has not changed the card error message.  :(

Any ideas?