I assume they are not on the computer.
First: Do not take any more pictures!
Second The only hope is with recovery software, check out the manufacturer of the card. Others might chime in with others.
Oh no! We are sorry to hear about the lost data.
So that the Community can help you better, we will need to know what model Canon camera you are using. Any other details you'd like to give will only help the Community better understand your issue. For example how was the data deleted? Have you done anything else to the card since this happened?
At this stage, your best option is to check the card out on a computer, I'm fairly sure you cannot do any recovery from the camera. You need an SD card reader - either one built-in to the computer, or one that is connected by a USB cable. If the card is readable at all, once the card is connected via the reader, it will appear in your File Manager as a drive.
I would first set the write-protect tab on the card to prevent any chance of further changes to it.
Are you using SanDisk brand of cards? SanDisk includes data recovery software for macOS and Windows systems that may still be on the card. If not, you should be able to download a copy of that software for your computer from the SanDisk web site.
If you are using another brand, you’d have to check if they provide any recovery software.
How important are the photos? If it’s once-in-a-lifetime images and you have had no luck recovering yourself, you would then need to look into professional data recovery services. Still no guarantee and they can be expensive.
Note to others that replied earlier… I think that recovery could still be achieved (professionals only) even if the card had a low level format done. That would be true if and only if solid state devices are similar to hard drives such that traces of old data could still exist even when new data is written. But this may not be true at all with solid state.
12/05/2023: New firmware updates are available.
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