tl;dr: BEFORE something happens, make sure that you set a known LOCAL password (not just a PIN) for your computer. Win10 setup won't volunteer to make that happen: you need to go into
Settings > Accounts > Sign-In Options and set (change) the password yourself. Then back it up somewhere safe, maybe even with a password reset disk if your computer still accepts such things. And if there is one in there already that nobdy
knows and you can't change, don't do anything that might bollix startup!
I had the same problem with a cheap little Win 10 tablet. Troubleshooting a Windows Update caused some issues, and it ended up rebooting BY ITSELF (I didn't ask for it, and neither did the Original Poster) into Safe Mode. Which of course wanted a local password
that I did not have, since the tablet was set up with a PIN and MS Account in the first place. Recovery consisted of:
- Download the correct flavor (32-bit in my case) Win10 to a USB stick using another computer (you have more than one, right?).
- Figure out how to boot from USB - not trivial for my tablet as it turned out (not just a simple BIOS setting, and even getting into the BIOS took some hacking - for AMI UEFI BIOS, try hitting ESC while it's starting). Eventually, after hacking around in
the UEFI shell, I discovered that after rebooting twice with the USB stick plugged in, the BIOS recognized it and offered it as a boot option (otherwise, it was just the internal SSD or UEFI shell). Isn't it fun when there's no documentation and you have to
go online (from another computer) and interpret the "hey try this" posts?
- Run the new Windows installation to the screen for for "Start this Installation", look for the "Repair This Computer" option, almost hidden in the bottom corner. Click on the Repair option.
- At this point, you get many other options. In my case, I had a recent Restore Point from before the update, and went back to that. You do have System Restore turned on, right?
- Took a while, but it's now unbricked.
Will need to spend some sysadmin time with the tablet now. Cleaning up stuff in general, and making sure that there's a KNOWN (and backed up) local password for the thing.
Oh, and BTW MS "answer" people: when the question is about logging in to Safe Mode (or even normal mode, really), an answer that involves pressing Windows+R and running MSCONFIG is a total FAIL. The Windows user interface is completely inaccessible
at that point. Read the question, please, and provide something more on point.