Option 1) The posts may not be lost. They may now be under the user “admin” unless you deliberately selected “delete users posts” when it popped up during the user delete dialog.
Delete Users
At the top, and bottom, of the table is the Bulk Actions pull-down box. In this particular SubPanel, only the Delete action is allowed. Any User that has the checkbox (left of User Image/Username) set, will be deleted when the Delete button is clicked. On clicking the Delete button, a Delete Users panel will display with a message “You have specified these users for deletion:” with a list of the Users you are deleting with the following question:
What should be done with posts and links owned by this user?
* Delete all posts and links – Check this radio button and click the Confirm Deletion button to delete the User(s) as well as any posts and links attributed to those User(s).
* Attribute all posts and links to – Check this radio button, select a User from the adjacent drop-down box, and click the Confirm Deletion button to attribute all the posts and links for the Deleted User(s) to the User you selected in the pulldown box. Upon successful completion of the deletion process, a splash message box near the top of the panel will display “x user(s) deleted”.
Reference: http://codex.wordpress.org/Authors_and_Users_SubPanel
Option 2) restore from your last database backup. (obviously this depends on when you last did a backup).
