From the web: Backup Encryption Key Prompt? (seems you may have downloaded an encrypted file....that's one possibility)
QUOTE from link above:
Based on my further research, you could run cipher.exe tool to display the encryption of foldes and files on NTFS volumes. To do this, please take the following steps.
1) Type "Command Prompt" in the search box and right click to run as Administrator.
2) Type the following command:
Then it will list encrypted folders and files if your system owns.
3) Please locate to the above encrypted folders.
For more information about cipher.exe, please refer to the following website.
BTW, after finding the encrypted folders, you may need to recover this files. I would like to share a link with you.
Recover encrypted files or folders
“Every atom in your body came from a star that exploded and the atoms in your left hand probably came from a different star than your right hand. It really is the most poetic thing I know about physics...you are all stardust.”― Lawrence M. Krauss
A 1792 U.S. penny, designed in part by Thomas Jefferson and George Washington, reads “Liberty Parent of Science & Industry.”