I have this problem on my HP Mini 5101 netbook. Fortunately, upgrading it was more an experiment to find out if Windows 10 would run reasonably well on this low-end device. It's a machine I rarely use.
The issue occurred after this week's cumulative update. I'm 100% certain that I'm entering the correct password. I have changed the keyboard language from "English (New Zealand)" to "English (United States)" and back again, entering the password after each
change. I've also enabled the on-screen keyboard and entered my password. Finally, I've entered my password using Caps Lock for upper-case, then used the Shift key for upper-case. No joy.
No problem (touch wood) logging into my main PC and Surface Pro 2. However, today my main PC came up with the Critical Error "Your Start menu isn't working"! I see that there's a lot more people experiencing that issue, so I imagine Microsoft will be burning
the midnight oil to get a fix out. Sigh. It was all going so well too.