For whatever it's worth, the last time I couldn't log in, I'd had to change my password, but my browser cookies retained my old password and tried to use it to login automatically. This caused me to be unable to get a login prompt, so that I couldn't enter my new password. I told my browser to delete site cookies for scrapyardknives.com, which corrected the situation by getting me a login prompt.
Also, I use Firefox as my web browser, and the "Web Developer" extension provides a toolbar that includes a "Cookies" button; it greatly simplifies managing your site and session cookies and is one of the best reasons to use FF. If you actually work in web development, another extension named "Firebug" is excellent for debugging javascript and inspecting a page's DOM elements.