The Spritesheets are a very good way of optimising your games. Some people consider it as a security tool for saving their game sprites.
But what happens if you loose the key to these locked sprites!!? Many indie developers have faced this issue for long time. In fact loosing data is normal is every game developers life.
Here are few tips as to what you can do in that case:
1: Talk to the support of the respective sprite sheet generator software. i.e Texture Packer, Zwoptex etc. Give them the authenticity of your sprites and they might just come up with a new project file.
2: If your project file was of Texture Packer then you could use one of our tools to edit your sprites in the spritesheet. You can extract or update the spritesheet directly within the software. http://store.bluegamerzstudio.com/downloads/spritesheet-plist-editor-for-texture-packer/
3: If your project file is of some other company like Zwoptex or Unity then you might just wanna try looking at our Spooky Any Spritesheet Extractor And Merger.
It works on powerful OpenCV and Java. It can Threshold your transparent PNG files. Which would give you access to your sprites in the sprite sheet. Also the same software could be easily used for Editing the same sprite sheet. Spooky is not 100% accurate as it works solely on Image Processing power. With better quality spritesheets, the results can be flawless.