Did you load something that is write proetected ? When a usb drive is write protected it becomes read only and that is something that is a setting that you can check to enable by right cloicking the usb drive in computer and selecting properties , then you select read only and that write protects the data on the drive. The only reason to do that is to protect the data that is on the drive from being erased or changed in some way but you can still see the data or in this case images.
Now you have stated that when you got the usb stick you filled it with images and then the next day you wanted to format theusb stick, that makes no sense. The images that you loaded the day before you no longer want ?