The question mark means Lightroom cannot find the folders any more, and you need to fix this, otherwise you cannot use these photos any more in Lightroom (you cannot edit them, you cannot export them, etc.)
To fix the problem, right-click on the folder, select Find Missing Folder, and then select that corresponding folder on the external drive.
When I restarted the external drive, all yearly folders were in solid and there were no ? marks anywhere. Miraculously, the problem has solved itself. How? I would not know ...
The result you are seeing is simple. When you restarted the external drive, Lightroom could now see the files and folders that it previously could not see.
Files on an external drive are expected to be, and commonly are, unavailable at times due to the drive being off/disconnected/sleeping.