This has never happened to me before. I changed my password, set for No Password on login, and, noticing that I wasn't an administrator, I made myself one, though I'm the only user and the only one with access to the box. I then closed the lid to my laptop thinking that would effect the change, but then neither the new nor the old password was accepted. I even wrote down the new one (and I never write down passwords). I tried various spellings, no dice.
After reading some other posts, I realized I hadn't logged out and re-booted. So since I couldn't logout, I forced the computer off. When it came up, it prompted for a password again.
I saw a post about Recovery Mode, so I tried that. Not sure it worked - I never saw a grub prompt (don't know what it looks like and didn't see any kind of prompt). But the laptop booted without a password prompt. When I tried to create an additional user (as the backup I shudda done in the first place), I couldn't authenticate with either new or old password, or an alternate spelling of the new one, in case I misremembered it. Still no dice.
But at least I had access to the GUI. After being idle for a while, screen went blank and on awakening, the password prompt was back.
My last resort, I guess, is to re-install. The installation is only about a week old and I haven't loaded much yet, but it's still a pisserbummer.
Or does anyone have any other ideas? : (
