Hi Elizabeth! There is a local group policy named "Deny log on locally" which prevents the guest login in Windows 10. You can check out this article for detailed steps to enable the guest account:...

Hi Luke! Boot your computer from Windows 10 installation DVD (or USB) and you can add a new admin account, by following this article:
Reset Forgotten Windows 10 Password without Reset Disk

After...

It's possible that the main administrator is disabled during upgrade. Follow these steps to activate it:

Press Windows + X to open the Quick Access menu and select "Command Prompt(admin)".
In...

The quickest solution is to perform a refresh on your problematic Windows 10 OS, I think.
Here is how to refresh Windows 10:
Refresh Windows 10 - Windows 10 Forums

It's easy to miss some steps when creating the windows to go drive manually. I suggest you using the freeware ISO2Disc or Rufus. Both of them support creating windows to go usb directly from Windows...

Pull out your hard drive and attach it to another working PC, you should be able to see your files and backup them.