Act fast. Lost data is not lost until it is overwritten, so the sooner you attempt recovery the better. The first thing you should do when you accidentally lost pictures from the card is STOP immediately using the SD. Because any write or edit may cause the lost data become "lost for good" simply due to
Normal method (Need a computer or laptop)
If your device is not rooted, use a third party computer program like Coolmuster or Recuva.
Install the program and connect your Android to your computer with a USB cable or your memory card with a microSD card reader.
Follow the prompts in the program - select where you want to search, what you want to search for and follow the individual program's steps.
Once you get your files back, save them somewhere other than where they were originally lost, in
case there is corruption in your device's memory.
Once you have them safely saved on your computer for example you can transfer a copy over to your device again.
If you're rooted, thing are a lot simpler.
Install an app like Undelete and follow the prompts. Nothing to it!
If, for some reason, you have root an Undelete is incapable or recovering your lost files you can simply follow the process above and use your computer.