TL;DR: As soon as you know images are missing stop using the memory card and only let a data recovery expert touch it.
*What do you do when you discover your photos have suddenly disappeared? Don’t Panic!
First thing is to STOP USING THE DEVICE from where the photos have been lost. If it’s your camera – turn it off! Take the memory card out and don’t use it. Ditto if it’s your smartphone or tablet. If it’s your computer, again do not use it. If it’s an external hard drive – eject it and turn it off.
Depending on how “valuable” the images are will determine the next step. Value can be financial and/or personal. Images created for business may have high commercial value. Family photos may be irreplaceable memories. If that’s the case contact a data recovery expert (or two). They have the professional experience and equipment that will be your best chance of recovering the images. Locally I know Xyber Data Recovery, for the glitterati there’s DriveSavers in the USA.
If you start taking more photos or browse the card on a computer there’s a good chance you’re writing over the missing images. That’s because when you delete an image it’s actually still there. The camera or computer is telling the operating system it can use the space and overwrite the missing (or accidentally deleted images).
Similarly if a memory card is corrupted most likely it’s just the folder structure that’s broken. Recovery experts and software can try to repair the folder structure or identify the image files and copy them. Photos including JPEG and camera raw files have a “fingerprint” that can be identified and recovered as long as they themselves haven’t been corrupted.
Your second option is to DIY and use photo recovery software. The application I’ve long suggested to colleagues is PhotoRescue. It never changes the contents of the problem memory card but rather creates an identical copy of the card and uses that to recover your images. Even images that are corrupted will be recovered and when cropped are sometimes usable. Download a free trial at www.datarescue.com. If PhotoRescue recovers the thumbnails for your lost images you can then buy a license for only US$29 to recover the actual images. If you know a good data recovery service or application let me know on Linkedin.
*N.B. All advice I give here should be used at your own risk – I won’t be held responsible for image loss! It comes from decades of experience and working with major camera, computer and software manufacturers as well as data recovery experts.