This exact same problem happened to me today, but I was thankfully able to resolve it and wanted to share in case it is helpful to others.
I was using my external hard drive this morning and everything was fine. This afternoon, all of a sudden, it was locked, and was requesting that I enter a 48-digit recovery key, even though I had never set up encryption or even heard of Bitlocker before, similar to the other posters above.
After almost having a meltdown, as I have years of research and articles saved in there, I did a google search for "microsoft support unlocking bitlocker." That took me to a page on support.microsoft.com that gave me different options, including "find my bitlocker recovery key". Then, from there, I logged into my microsoft.com account, at which point it asked me to verify my account by retrieving a code sent to my email account. After that, it took me to a page listing my devices and corresponding recovery keys. Each recovery key had a "key ID" next to it. I looked for the one that matched the "key ID" that was listed in the prompt to enter the recovery key. I pasted that recovery key into the box. I had to do it several times before it worked, but it indeed unlocked my external hard drive. (I noticed that at least one time the paste function did not paste all 48 numbers so I manually checked to make sure all 48 were there and then it worked.)
I note that, even after unlocking it, the external hard drive locked itself again a few minutes later. I was able to unlock it by entering the same key again. I'm not sure how to stop the drive from locking by itself in the future, so will make regular back-ups of the contents of my external hard drive.