Hello JasonR,

From what you're describing it seems that someone has added themselves to the securities group located in the workspace.  If a user has added themself to any of these groups this will lock out the remainder of users who can access workspace.  I would check to see if someone has added themself to one of these securities groups which is most likely causing the issue.  There is a way to find out if someone is in one of these groups.  If you look in SQL at the Workspace database and open the dbo._ActionPermissions Table this will typically list the users who may have been added to the groups.  Deleting the user here will NOT remove the permissions but will give you insight as to who may need to remove themselves from the workspace securities groups.

 The users found in here should be able to log on to workspace and access the securities tab and remove themself from any groups they may be in.  If you cannot find a user or who could have added themself there is a script that will wipe away all user permissions for these groups in workspace.  Please note, making changes to the databases is not supported or recommended in doing this in a production environment it should only be done in a testing scenario.  before making any changes to the databases please back up all of them to ensure you can roll back if there are issues.  Please let me know if this helps and if you need the scrip we can find a way of getting that to you.

Hope this Helps,

Jon

There is a Knowledge Base article on this subject, too.

Please see the K2 Knowledge Base web site, and see KB article 291:
http://kb.k2workflow.com/Articles/KB000291.aspx