Agree with the above ways. I would also check the spam content. Many people will stop posting if they see too much spam, but of course this is not the only reason forums go dead. What is the topic you are covering?
This really depends on the forum and niche. Not all forums have what it takes to be brought back.
I would first look at the different posts and see if there are any active members. I would give it a huge push with maybe different style and change a couple sections. Host a contest to get it going and email everyone to join. I wouldn't mind buying some posts at the very start to get it going either. You never know the people that are posting to get paid might stay for the contest and afterwards as well!
One of the first things to do when deciding to revive a site is figure out why it died in the first place. If you don't do that there will be a high likelihood that your site will just die again. (Probably for the very same reason it did the first time.)
If your site died due to toxic staff member(s) then you have to boot them out. If your site died due to staff inactivity then you have to ensure that you're really ready/able to resume. There are plenty of more reasons why sites die, but I'll just go with the above two first.
After you decided to revive a site then you actually have a lot of work to do. You have to comb through any old content and see what's worth keeping and what needs to go. Look at your rules, your FAQ, everything in your official documentation. Spend time cleaning up your documentation by removing useless rules or rules that are covered under others. Watch your wording in rules too (stay away from "please" and "try" since those make it sound like there really is an option to do otherwise...)
It's typically a good idea to create a new skin when you revive a site too since then it looks nice and new too. Plus people who see the site again are more likely to stick around if that site looks like it's changed and improved. And a shiny new skin can also help improve your own motivation and interest to be there.
Also, don't forget about your content... Spend time really creating more. A lot more. Any chance you can? Create a new topic. (I don't recommend creating duplicate accounts to post since that's tiring and deceitful, but definitely look into post exchanges and post packages.) Also be wary of your content quality... I mean it's pretty easy to spam up topics like: "what's your favourite game" or "who's your favourite actor" but those topics generally are pretty low quality with short (possibly even less than 5 word) replies. Instead look at content that's more unique and interesting. Ask questions that invite more thought out answers. So instead of asking what games a person likes, ask them why they like the games they do.
Then when it comes time to advertise, really push your site and show why your site is better than all others. I mean if you're not confident in your own site, why should we (prospective members) even consider looking at it? There are thousands of other sites out there with a vast range of content. If you're not 100% sure on yours then we will pass on your site.
Also, use a myriad of different sites. I know FP is great, but it's only one site and one circle, you need to really branch out to reach more people. There are tons of other sites which will work to help bring more people to your site.
Similarly, you really need to be enthusiastic about your site. Since most people don't want to join a site where the admin is a downer, spending time moping or complaining or worrying about numbers. Which ties nicely into my next point... Success is measured differently by different people. Don't worry about having huge numbers since those can take a VERY long time to get. (And also... don't look at competition sites, that can be very depressing.) Instead just focus on improving your site and making it the best it can be.
That's all I can really think of for now, hope some of that helps.
Alright, you have a problem, but also an opportunity. As mentioned above, you need to revamp the site...in more ways than one. Now find 1-2 staff members who can help you run the place and inject some activity.
But really, promotion is the key. As mentioned above, become competitive, get smart with the promotion. And look at other methods not just forums. As well as forums look at link directories, search engines, topsites, social media and more. Make sure your promotion is well thought out, don't just spam everyone with poor grammar and illiterate sentences, make sure the advertisements you use are catchy, and get people interested in the site.
That only works if those users had subscribed to those topics. Not everyone has their subscription settings to follow every topic they're in. And some forums don't do that as default behaviour either. So they might have to actually manually set to subscribe before they get any notifications.
You could always try a mass email letting people know your site is alive and well, but in my experience those don't really work that great. People that leave might have a look again but they probably won't stick around any length of time. (They may have moved on to other more active sites.)
Instead, I recommend following the advice from my previous post. Fresh look, more unique content of higher quality and don't focus on numbers/compare to competition. Just keep focusing and improving your site.
I think cleaning up certain topics, boards. A new theme/design.
Make a bunch of new posts, get some friends to help post and post exchanges are good too. Then you just start advertising your forum around.
I usually partake in more exchanges, that way it helps to balance out the activity and I send auto messages to any "inactive" members through a filter.