You should check the db to see if things match up there. In the db you will find this under the wa_settings table, in the name column look for admin_username and admin_password, then check the value column for these two records.
One other thing you can try is to use another browser to see if you are having a problem based on the browser.
Post back and let us know what the result is, if we need to open a ticket I can do that.