Saving your password and username is a browser thing, it's not done by this site.

If it's no longer being saved, then you might have cleared your cache and/or cookies recently that caused the browser to no longer save the information. I never save that kinda info for privacy reasons, but you can always clear all your cookies, cache, history, whatever and login again and see if the browser saves it properly.

The easiest explanation is that Cookies are small files of data that are usually created/saved by your web browser. They allow for the ability of some data to be stored or to allow webpages to tailor ads or other content to you based on your previous visits to the site or other browsing you've done. http://www.whatarecookies.com

For some browsers, clearing history might also remove cookies depending on what your settings are (it may even be a checkbox when you click on "clear browsing history"). If not then tutorials online will more than likely answer a question of how to clear cookies for whatever browser you use.