Did you clear your cookies and your cache in your browser?
Also when you go into the AdminCP you might want to clear the cache in there as well...
Maintenance>Clear System Cache
It very well maybe from a plugin sometimes they act up at the weirdest times.
Have you tried to globally disable them and try to login again after you clear your browser cookies & cache?
Open your config.php file and find:
Add below...
Code:
define('DISABLE_HOOKS', true);
Save and reupload.