@Gage-Pritchard The supplied Duet SD cards often have two partitions on them, one FAT32 partition called 'boot' that should be readable on a Windows PC, and one for booting a Raspberry Pi that is not readable by a standard Windows install. This message might be due to that second partition. Make sure you're looking at the correct partition. Look at the drive in Windows Disk Management, and see if it has other partitions.
If not, then it looks like the SD card has failed, or something has caused it to fail. Do you have a backup of your configuration files?
You can try and format the card, to see if it recovers the SD card back to working condition. This page has details on SD card specification, formatting, folder structure and required files, when rebuilding the SD card from scratch: https://docs.duet3d.com/en/User_manual/RepRapFirmware/SD_card
Ian