My images suddenly vanished

I have had success with this... Download this camera photo recovery program
http://www.facebook.com/pages/Asoftech-Photo-Recovery/126127714100611?sk=info

You have to use a card reader either on your computer or an external USB card reader. Put the card in the card reader, open the recovery program, then select the drive where files are deleted, then press "Start" and wait for it to do it's magic.

Format won't make the data in the card (of your Canon camera) deleted permanently. Try some photo recovery software to rescue the pictures, here are some for your option.
Photo Recovery (for Windows)
Photo Recovery for Mac

Before your pictures are recovered, do not attempt to save more files to the card in case the original files(your pictures) are overwritten.

Ok! Next time, I hope you can be more careful about your camera and always get accustomed to back up all your needed photos well. But, now what you should do is to act carefully for restoring all these photos back as below:
Firstly, stop using this camera immediately in case that any new file would overwrite your original photos and make them gone permanently. So, you should take care!
Secondly, plunge this camera to your computer and prepare to retrieve all your deleted pictures.
Thirdly, apply a deleted photo recovery freeware to retrieve all your deleted photos back successfully:
http://blog4mark.blogspot.com/2012/12/photo-recovery-after-delete.html
I believe this data recovery freeware can work perfectly on your camera memory card just in the same way as it completely retrieves all the deleted data of my friend off from his phone memory card. Just give it a shot!
Fourthly, save and back up all the restored photos on a different camera memory card or drive in case of data recovery failure.
Note: Never forget to back up everything important at least on two memory cards or locations in the future.
Good luck!