The plugin / plugin.php has been deactivated due to an error

Were you either using the MailPoet or Slider Revolution plugins? If so, see http://blog.sucuri.net/2014/07/mailpoet-vulnerability-exploited-in-the-wild-breaking-thousands-of-wordpress-sites.html or http://blog.sucuri.net/2014/09/slider-revolution-plugin-critical-vulnerability-being-exploited.html

Otherwise, remain calm and carefully follow this guide. When you’re done, you may want to implement some (if not all) of the recommended security measures.

Yep, that’s a hack. See my first reply above.

You’re welcome, good luck with the cleanup, and make sure to keep regular backups once you’re done!

http://codex.ww.wp.xz.cn/WordPress_Backups

You should be fine to use the old database.

Also, when removing old files, don’t remove anything in /wp-content/uploads/ (that’s where all of your uploaded files, images, etc. are), but do look through there for any suspicious files, especially .php files (which shouldn’t be in there at all).

You’re welcome!