just to make sure i understand; everything was working properly 2 days ago; and then yesterday it stopped working? were you attempting to do an upgrade from v1.5.3 to v1.5.4? or are you saying you did nothing, and now you can no longer get to the admin page?
if you are saying you did NOTHING, that would be very curious. some one could have hacked your site.... your hosting company may have changed something... who knows... i think the odds of that happening are slim, but that said the troubleshooting remains the same.
if you were upgrading, then you did something wrong in the upgrade. if you could get to the admin page previously, and then changed the directory/folder where the admin was, and now you can not get to it, that would indicate a problem with the configure file where you need to make changes to accurately reflect what your folder name is. the 2 variables in there are:
define('DIR_WS_ADMIN', '/ADMIN_PLACE/');
define('DIR_WS_HTTPS_ADMIN', '/ADMIN_PLACE/');
more than that, you would have to start looking at the access logs or perhaps to using the developer tools to see if you are getting redirected to somewhere else that does not exist. i have found .htaccess files at the root as well as in the admin folder so perhaps one of those is adrift...
good luck.