First of all, photos should always be downloaded immediately and then backed up.
You should not be storing photos on a memory card.
It doesn't happen very often, but memory cards can and do fail.
When you simply delete (instead of format), it can cause errors on the card.
Regularly formatting them in the camera helps to prevent problems.
For now, try a USB card reader in the hope that you can get the photos off,
then remember the rules for the future.
If you can't get them off, google for some image recovery programs. There are plenty of them out there, but strangely enough not everyone seems to always work, sometimes it takes several programs to get the photos back, but you should have a reasonable chance of getting at least most of them back.
Good luck!