Quoting Jamie Holly hovercrafter@earthlink.net:
Try emptying the sessions table in your database, then login again.
You may also want to try and clear all the cookies in your browser for the local domain.
Also make sure the files and tmp directories are writable and not full. Login issues of this nature can happen if the sessions cannot be saved.
-- Earnie http://r-feed.com Make a Drupal difference and review core patches.