Same exact problem here. Fixed it by doing this:
1. FTP/SFTP to site (or use some file manager, cPanel, whatever).
2. Deleted wp-content/plugins/wordpress-seo, the whole directory. (Or to be safe, renamed and downloaded it first.)
3. Then I could log into the Dashboard and re-install the plugin.
4. Deleted .maintenance in the main wp directory.
So basically, this: http://wordpress.org/support/topic/30-stuck-in-maintenance-mode-after-failed-plugin-upgrade
Wonder if this is a bug in WP, the plugin, Softalicious, whatever. Do you use Softalicious?
Several files in the plugin were truncated or missing:
*admin/class-metabox.php
*admin/class-sitemaps-admin.php
*admin/class-taxonomy.php
*changelog.txt
Plugin version: 1.4.22
WP version: 3.8
Softalicious: yes
By the way, I found the offending plugin by looking for an error_log file in wp-admin. You can also try setting define('WP_DEBUG', true) in wp-config. One of those techniques may be helpful, depending on the host.
