Your SD card may be corrupted, In which case you may have lost all files. Never pull any stroage device out while in use without safely ejecting it first. There isn't much you can do to fix this apart from tryign programs like DiskDigger to see if you can recover your files. Here is how to safely eject (unmout) the drive
HOW TO SAFELY EJECT A STROAGE DEVICE (Win 7,8 & 10):
1.) Go down to your System Tray on the taskbar (next to where your system clock is)
2.) Find the 'safely remove mass storage device' icon (looks like a USB stick)
3.) If you can't see it, you may have to click the upwards arrow to view hidden icons to find it.
4.) Once you've found it, right-click on the icon to bring up the options menu.
5.) Find the device you want to eject (e.g SanDisk SD Card) and click on its name.
6.) A pop-up should appear witin a few moments saying it's safe to eject it.
7.) If an error comes up, close any programs that are using the device and repeat the first 5 steps.