It is very common and wide known error, can be caused by multiple things, yet most probable cause is not very easy to find, most probably you will find some tip on how to make system restore or reinstall whole OS.

In short - upon reboot you see black screen and mouse cursor. You can even get to control panel using sticky keyboard feature (multiple Shift presses and clicking later on Control Panel path), run Explorer and such.

Most fun thing what most frequently it can be not related to your computer, at all. If you have network mapped drives it is enough for computer who provide them to have some issues, usually it means what Windows is able to find it on network, but is unable to communicate with it properly. If such case happens Windows wait for up to 7 minutes before giving up on attaching network drives.

How to find if this is the case, and not some other stuff? Check both Safe mode and Safe mode with network drivers enabled, if first load perfect and second has same issues - it usually means issue with network drives.

Check in more detail - https://blogs.technet.microsoft.com/markrussinovich/2012/07/01/the-case-of-the-veeerrry-slow-logons/