Hello, Tinamarie!
Whats confusing to most of us, at least me, is you keep using the term "transferred". To me, that means "moved", so we just might have a contradiction in terms here and it's confusing to give you assistance with your initial problem that your data is no longer on the SD card because we don't know the steps you went through to get the files on your PC.
I understand that you just connected the camera to the PC with a USB cable, and that's fine. The PC should see that as a drive with a name and allow you to move, copy, or delete images on that drive. Sorry if I describe stuff you already know, but your methodology is pretty vague TBO. Anyway, to Waddizzles point, you should use your right mouse button to perform any action of copy/move because it will not perform the task unless you select the action from the menu that appears when you release the right mouse button. Another good option is to use the menus in file explorer to copy, move, or delete. Drag and drop, although convenient, can cause various problems like misplaced files. As an X systems administrator, I had to deal with this on a weekly basis as users would drag files and they would mysteriously disappear. Although I would eventually find the missing files, it required a search of my server only to find the file in the wrong folder or some such. Not that this is your problem, just an example of the many things that can go wrong with drag/drop, if indeed that is what you did, we just don't know.
As to John's (jrhoffman75) point, the card may have been corrupted if you didn't eject it using the windows command to release the drive/device. You may get away with this hundreds of times with no ill effects, but it just takes one time to corrupt the cards memory. By not ejecting the device in Windows, you could interrupt the process of writing info to the file allocation table (FAT) which keeps track of where the clusters of memory are that store your files/images. I'm speaking from experience here, as it's happened to me with an external HDD.
As to recovering the data, there are several on the market. Most have a free trial and will either recover a certain amount of data or will show you what it recovered and want a fee for the software, it's usually inexpensive and they offer different price structures for home or pro use. I've used Rovoverit, and it did a good job on one of my external drives.
I'm sorry if I've covered stuff you already know or do, but as mentioned, your methodology is a bit vague, at least to me 🙂
Newton
