How would you bring users back?

I’ve got this other website with several thousand registered users.  This morning I wondered – how many of these users are really active users and how many registered and then abandoned their account.  I was kind of shocked to discover that for one of my target demographics, only about 20% of those registered users visited the site in the last 180 days.

What I need to do is bring those users back.  Do you have any suggestions?