First thing is do not do anything else to the camera or USB drive. Do not take any pictures or save any files.
There are a number of programs available that will let you recover files from the memory card or USB drive. Most cost, but at least one is free. The one I have used and worked for me is free and is Recuva from Piriform.com. Download and install this program.
Start the program. Next, remove the memory card (SD card) from the camera and put it in your computer or card reader attached to the computer. Then, you will follow the steps in the program to pick the location where the pictures are (the memory card or USB drive will appear in your drives on your PC). You will choose the files to recover, and if you want them recovered there or to another location. The program will then recover them, however, you may need to change the first letter in the files afterward, as it was probably changed.
You will do the same process to recover on a USB drive. Piriform says they can recover files deleted from the Recycle Bin also; just do not delete anything or run anything on the PC if you realize you accidentally deleted files until you do the recovery, as new stuff could be written where your files were, and they cannot be recovered.
Send your questions to Dwight Watt at email@example.com. He teaches at a technical college in northwest Georgia and does consulting work for businesses and individuals. His website is dwightwatt.com.