Sorry to hear you're having problems with the login. Sounds very puzzling indeed. For security's sake, one thing you can do is limit access to your WP-admin by IP address - at least temporarily. Then only you can access your admin and work on your site in security. Check out this article for information on this issue:
Brute Force Attacks
Look for "Limit Access to Wp-admin by IP".
If you're clearing you're cache and you're trying to use the password that your browser has saved, it will generally be cleared and you'll have to re-enter it. However, you did say that your password is disappearing after you type it in and BEFORE you hit the enter key on the login page.
I'd start with limiting the login so that you're not getting hacked while you're figuring it out. One quick thing you can also do is make sure to check and repair your MySQL databases - you may need to check your host for that. It doesn't hurt to do this just in case the database has some problems.
Make sure that you're going to your login page (double-check the URL you're seeing when you login). Check your .htaccess file for any re-directs that look suspicious. If you only have your WordPress installation then it should it should look like the one in this post:
If that isn't the issue, start simplifying your installation.
If you're running a bunch of plugins - make sure that're not having anything to do with your login issue by disabling them. Strange things can happen if you have a plugin is going awry.
If you continue to have the problem, then provide us more information about what you're seeing. Please post any error messages if you continue to have the problem. We'd be happy to try to help!