The rough answer is that you have too many styles right now and removing one will be the simplest way to resolve it.
If you can't get into the admin CP, you can turn on debug mode in library/config.php:
$config['debug'] = true;
You should be able to temporarily get back into the admin CP to remove a style. (Note that styling may be broken.) You should then pick up with the upgrade if that's what you were doing. Be sure to remove debug mode afterwards.