Mike and Ramskie, here is a LONG list of suggestions, all of which are easy to do.
Go to the sign-in screen and let it sit for a couple of minutes. Then try again.
For some weird reason, this sometimes works. In the bottom right corner there should be "ENG" for your keyboard. Click it to make sure it is using the right English. If it is, try changing it anyway--i.e. from US to UK or vice-versa
Make sure that you are not set to Caps Lock.
Try using the password for other Microsoft accounts. Specifically, try the password to your e-mail.
Try entering through Safe Mode. There is a Power Button on the sign-in screen that will help you do that.
Try using a different keyboard.
If you had a previous password, try entering that.
The problem might be the @ Despite what your keyboard says, sometime it is on 2 and sometimes it is on Q. Try using the other one. You probably have to press AltGr at the same time. Try pressing shift at the same time.
Does it say "I'm not..." in the lower left corner? If so, click it to see what it tells you. It might be stuck on an update and think that Windows 10 is not yet installed.
Sometimes this works. Restart your computer and keep pressing F8 until Advanced Options appears. Refresh or re-install Win 10. You might have to press F9 instead of F8.
Does it say "Your Name" in the bottom corner? Try typing in your account name.
Turn off your computer and unplug the power cord. Unplug all devices, including your mouse. Plug it in, turn it on, and see if it either installs upgrades or finishes re-installing Windows 10.
Try typing in the name of your computer. I don't know how this works, but on the sign-in screen, see if you have the actual name of your computer; if it is wrong, try to change it.