If you do not want to loose content, meaning restoring the backup, you can have that member re-register, or create an account manually for him/her, then you can re-assign their posts/topics with a few sql queries.
Not natively, unfortunately. You could restore from a backup, but then you lose any changes between now and the time the backup was taken, or manually recreate the member. All of the member's posts and other content will now be associated with no account (guest).
Beyond that, you'd be delving into a dark journey into recreating SQL rows and foreign keys, and that's outside of the scope of what would be safe for a novice to attempt. If you have a backup to compare against, that would probably make this easier to attempt.
MJG LBA reacted to Miss_B in How to Restore a Deleted Member