I am in the process of bringing Just BS back to life. It was a really active community until I closed it.
It's hard. Takes a lot of work. The main thing is to let the dedicated users know by sending out an email. If you can convince them that the site will not close again they will help bring some of the other members back.
The primary issue is dealing with the fact that many potential new members will come to your site, see it is filled with old/outdated content, and not even bother registering assuming the forum is dead/dying. For this exact reason, if I were ever going to consider reviving an old dead forum I'd make absolutely certain that I'd filled it up with fresh new content beforehand and it wasn't filled with easy-to-use years old threads that could be off-putting to potential members.
Alternatively, you could be a bit of a cheat and just manually amend the database to add x amount of time to all post creation dates. Essentially if you were using a year-old database you could run a database query to update all of the post dates one year forward, to make the posts seem recent. This won't help with the issue of getting people to return to the forum directly, but it just might be what you need to bring those people who do check your site out back as members and encourage them to participate.