Android Smart Phone Blues...

Try turning off the phone and taking out the battery..this fixes alot of problems sometimes...

Add a free file manager app such as Astro. It will allow you to explore all directories on the phone, to search for the pic files.

Doesn't the "One M8" have an external Micro SD? Have you taken the card out of the phone and directly to the computer? If the pictures were stored locally on the phone, try connecting the phone directly to a Windows computer. You might need to enable USB debugging. Some phones vary on this point. Use your computer to look through all of the folders.

Dropbox is very handy for many things. Picture backup is one of them. If you create a shared folder on your home computer, all data backed upto Dropbox is immediately available on the home computer.

That is odd, as I've never heard of photos getting zapped off a phone before. I assume this is a HTC One M8?

I personally use Dropbox myself. That way, I can swap between my iPhone 5S and my HTC One M8, and have access to the pictures. I also back up the Dropbox directory to a local drive because I've had sync errors toast my entire cloud drive before.

Are you sure they didn't get moved to another folder or location...go to "Folders" or "My Folders" and poke around.

Dropbox is free and easy to use. Auto download and you can edit or delete later if you don't want a photo.

If your Android stores the pictures on an SD card, they are usually completely recoverable. Purchase a cheap recovery program such as Image Rescue, and you'll be surprised at how well it recovers them.

I've had the same problem with an iPhone, I began using Picasaweb from Google and use an app that automatically uploads my pictures at GPS coordinates that I have set up. For example one of those is my house so if I've taken any pictures, when I pull into my driveway they automatically upload to Picasaweb.
It might be an option for you in the future.

Feel your pain. I have had things disappear too. One suggestion is to set up a cloud storage, Dropbox, whatever, and have all your photos backup automatically as soon as you take them.