If you just did an upgrade to magento or for some reason you try to log into the admin and you cant...and you do not get any errors...its more than likely a caching issue.
remove everything from the var/cache folder
Get to your root of your magento install
cd /var/www/magento_root_location_on_your_server/
rm -rf var/cache/*
Dump your browser cache/history and try again..OR just try a different browser, I bet it will work.