SOLVED Password No Longer Works!

[PowerBattery]

This is a Windows 7 Dell laptop. I hadn't used it in awhile, so I fired it up. It worked for a couple of years with no problems. I changed the battery. I got more use out of it. I put it in another part of my house for convenience. When I fired it up, it wouldn't accept the password. I didn't change it, but in the event that I had changed it, I took a guess at a few probable password changes, but they didn't work. What could have happened?

I restarted it and I got the password screen and tried it several more times. I restarted it several more times while holding down different F keys, but got no sort of screen that would allow me to check for problems.

Any idea what's going on?

 

[PowerBattery]

SOLUTION! I continued to start the laptop and turn it off. Eventually, a prompt at the bottom right of the screen appeared and I pressed f12 until I got a diagnostic screen. I followed the directions. It scanned about 10% of the hard drive and asked whether I wanted to continue scanning. I chose Yes. After it scanned the machine, it returned to its previous condition. I can now log in with the known password.