Hi all!
Today when I woke up I've had the same issue. All my Apple devices were in lost mode. My iPhone was under my control again after changing my iCloud password, my iPad needed to be factory restored and my Macbook Pro was aking for a 6-digit code. I've tried all my known combinations, but nothing worked.
After that I've rang Apple support and was transeferred to a senior consultant, he could only tell me that I should bring my Mac to a premium reseller. They were not able to solve this issue by phone. I was told that I should bring a proof of purchase, a valid ID and some money, because the fix is not for free.
Because it's sunday, the best option was visiting one of the Apple Stores in the Netherlands. Unfortunately I live in the south and driving to Amsterdam back and forth takes around 400 km. A better option was reading all relevant posts on the internet and try to find a solution myself. That turned out to be very hard. Almost every single topic tells that you have to visit an APR. I've also found some very obscure Youtube clips that are made by hackers to make some extra money out of this. Don't spend a single penny on those fraudsters!
In one of those topics I've found a post from some guy who claimed that he totally drained the battery, made a restart and his problem was solved. Since I had the whole day of trying things I thougt it doesn't hurt to try. I pulled the power cable out of my Macbook and started waiting. Because it's a 2016 touch bar model the battery lasts very long but by the end of the day my macbook shutted itself down. Before that happends a battery indicator pops up. Then I didn't touch my macbook for another hour. After that I've plugged in the power cable and powered up my mac. And what happend after that? My normal login screen showed up and was able to gain access again. None of my files was lost.
To be sure I've logged into iCloud and went to Find my iPhone. My Macbook showed up unlocked and wasn't reported as lost anymore. I would strongly recommand everybody to try this first, before going to an Apple store.