All that information ruled out many possibilities.
Now I can make an educated guess as to what happened,
and can tell you what not to do in the future.
The only chance you have for retrieving some of your pictures is with file recovery software.
Their are hundreds of such programs and they all do just about the same thing.
So I would not spend a lot of money to buy one. you can find free programs.
One such free program is RECUVA, you can do a Google search and find it.
You will not be able to recover all of the pictures.
What happened ... most likely:
Somehow you wrote something to that memory card with your computer, another camera, or another electronic device. That can also happen if you used the computer to delete any picture file on the card.
If something other than the camera writes to that memory card it must also write a File Information Block (FIB) to keep track of where it put the information. Then you have two FIBs on the card.
One by the camera and one by that other device.
The two FIBs do not know the other one exists.
The camera FIB can overwrite data that the other FIB wrote, and vise versa.
The result is corrupted picture files and missing files.
For the future:
If you buy a new memory card - Format the new card with the camera before you take any pictures.
Do not use the computer to put more pictures on the memory card (you won't be able to see them with the camera anyway).
Do not use the computer to delete anything on the memory card.
All editing must be done with the camera.
Do not put the memory card in another camera or electronic device.
People have been known to put their memory card in someone else's camera so that person can look at their pictures. Don't do it.
Back-up:
Many people keep all of their pictures on a memory card until it gets full.
That is OK only if you have another copy somewhere else.
After a days shooting, you should download a copy of all of that days picture files to your computer.
I put mine in a folder whose name includes the date and event.
(i.e. 20120414birthday).
That makes them easy to find later.
Each month you should make a copy of all picture files on your computer and put them on a CD.