Welcome to the forum! If the pictures were saved to the SD card and not the phone itself, they're still there. It usually takes a while for a phone's gallery to rebuild the thumbnails of pictures it finds on an sd card.
If they weren't on the sd card, that makes things more complicated.
You can download a file explorer app to the phone to search for them. Some popular ones are ES File Explorer and Astro File Manager. Or, you should also be able to hook up to a computer and use it's file explorer.
The folder you'll be looking for will likely be called DCIM and may be within other folders like "Camera" and/or "Android," but if you set the phone up to save them to another location, you'll need to know that location. This, of course, only works if the pictures were on the SD card to begin with.
As
@dezymond suggested, it's a great idea to setup auto-backup of your pictures. Google Photos will allow you to do so with unlimited storage of "High Quality" images. Dropbox, Facebook, and Microsoft OneDrive also offer photo backup options.