I have the same problem. I installed the Headers and Footers plug-in and now SEO by Yoast has totally disappeared.
I tried reinstalling SEO, but received the following error notice:
Destination folder already exists. /mnt/web1/e0/63/51425163/htdocs/XXX/wp-content/plugins/wordpress-seo/
Plugin install failed.
Do I have to delete the destination folder and try again? I have already deactivated Headers and Footers and deleted it.
