restore your database from the backup you made before you upgraded and then follow the instructions here: http://community.elgg.org/mod/groups/topicposts.php?topic=438396&group_guid=212067

Basically, you must have modified how Elgg talks to the database so the upgrade script fails. If that doesn't solve it, put a detailed description of your problem and what steps you have taken here: http://trac.elgg.org/ticket/1928

Abrikos, look at this plugin. I'm 80% sure the gpl version allows you to send internal messages to all users. The commercial one certainly does. There's also some more advanced mailer plugin here, though gpl version is not ready yet.

help !

Please use the search in the plugins sections:

http://community.elgg.org/pg/search?entity_subtype=plugin_project&entity_type=object&search_type=entities&q=send+email+to+all+users&category=all

The first result does what you ask.