Have you stored new file on the same memory card?
The new data will rewrite the original data and wipe it forever.
Therefore, as long as you have not rewritten your wanted files, you do have the chances to recover your data. You could try a free memory card recovery tool that has ever helped many web friends retrieve their precious photos, videos and files, including me. So, you could give it a shot. Note: After the recovery process, you should format this card to make it work again.
You also should learn a lesion to do regular data backups in the future.
Most of the time, error like not formatted is causes by virus on the memory card. so first thing you may do is to use free anti virus software to scan the memory card and make sure that it is clean. You may use Microsoft Security Essential or AVG Free http://goo.gl/r2w2uK http://goo.gl/byvPsq
After you making sure that the memory card is virus free, you may try to get the data from the memory card, see this guide http://goo.gl/gfSfaA
If the memory card is detected by computer, or not shown as drive letter on computer, you may assign a drive letter to the memory card in device manager http://goo.gl/B7crmP