I have a canon powershot sd790is digital elph abd

No they should still be there, but you're going to need recovery software, and an external card reader to recover them. Take no more pics or videos until you've recovered the photos, and take the card out of the camera. Here's some freeware photo recovery software that should be what you're looking for.
You will need to use a flash card reader with this software. If you don't have one, get one, they're extremely inexpensive these days, and they actually are a better way to download your photos than directly from the camera.

The reason this occurred is that you likely never formatted the card. The card will work fine in the beginning but eventually will corrupt. AFTER you've recovered the photos, format the card in your camera to prevent this from happening again (dig out that instruction manual to figure out how). Keep in mind formatting will also totally erase your card, so again only do this after you've recovered the photos.