Plenty of options posted here for image recovery. In the future it's always best to reformat the memory card after you copy the images off. Dont just delete them. This helps with a lot of issues including recovering images from the card should something happens to it.
A regular format of a card does nothing except mark the images as deleted in the fat table, no different than deleting them individually. A Low level format must be done to actually clear a card. The option to low level format a SD card exists in most recent Canon cameras, but a low level format or erase of a CF card must be done in a computer.
Excellent clarification. Yes SD cards require a low level format, not just a format. Otherwise the camera must erase the memory location prior to writing data to it. If the card is low level formatted, the camera can skip that step. One could assume that over time the card could become quite "noisy" and cause problems, even a readability problem.