When you insert in in your computer, does the computer detect it and say it needs formatting? Or can you not see the card atall?
Sounds like the cards have reached the end of their life, time for some new ones.
Try formatting them, you can get software to attempt to recover any data on them.
Check the type of SD card you are using. SDHC cards can only be read on HDSC card readers, but SD cards can be read on SDHC readers. If your pooter has an inbuilt reader and is a few years old, this will probably be the reason. If not, try a new lead for your reader.
You should also format the card in camera before it's used.
You could try ZAR as a free download and see if that can locate any photos you've taken.
------------- Some days you are the dog,
some days you are the tree.
Has your data disappeared or become invisible,check properties to see if the card is empty.
There are ways of recovering invisible data.
Always back stuff up in several places.