If you use Windows 10, you may have already installed the recent massive update. If you haven't installed it yet, don't do it until you've got about 45 minutes free for the update to prepare your machine and install on restart.
One wrinkle you should also prepare for: if you use a local numeric digit login, the update will automatically switch your login to your Microsoft account password without telling you that's what they've done. So when you attempt to login after the update, you'll be told that your password is wrong. Just make sure you know your Microsoft account password before you update, unless you're already using the same password as your local password. Also, under the passcode box, you'll see two icons. The left icon is for numeric login, which may not work, and the icon on the right permits alphanumeric login using your Microsoft password. Be sure to click the right icon, if the left doesn't work.
Why did Microsoft do this? They're trying to get everyone to be all Appley and Cloudy and get everything merged. If you wish to switch back to a local numeric login, you can do so somewhere in settings. I'll probably do that because it's a little more secure, not stored on a remote server, but use a high numeric code. Up to 10 digits is possible.