unable to login to my development site.. no idea why

i have been working on my local site here (v 2.1.2) and while making some CSS changes i found that i was not able to log in to the site at all. every time i enter the login credentials i am taken to the login page with no error messages being output anywhere. i have added the folder 'disabled' to the mod directory and then run upgrade.php to clear the cache of all plugin information and i can view the site in it's basic form ok, but am still unable to login. there is nothing relevant in the logs! i have never seen this behavior before and have no idea what the cause is. i have run a repair on all the database tables in phpmyadmin, but that made no difference. anyone seen this before?