POOF! Overnight your website traffic collapsed. After a day or three you might come to realize Google hates your website and has (for whatever reasons) completely de-indexed your website. If your domain/pages have been completely banned, you ought to swallow your pride and give serious consideration about what to do next.
After you've ascertained there wasn't some major hosting outage, temporary glitch and checked your Meta Header section and Robots.txt file to make sure no errant 'rel=noindex' accidentally told Google not to index anything: You're left with some big decisions. What do you do if you've discovered Google has totally de-indexed your site instantly and permanently?
1. Be realistic. Your chances of recovery are slim to none. You didn't just trigger a lesser Panda / Penguin penalty - this is total de-indexing. If you HAVE to try to salvage a completely de-indexed domain, you can try to do a comprehensive link cleanup and disavow. And wait. And hope. And pay the 'opportunity cost' lost waiting for some reversal that may never happen. Personally I've never bothered: Time IS Money -- Lost.
2. Where do you stand with Bing / Yahoo / DuckDuckGo? Type site:yourdomain.com into each of these search engines and see if some of your pages show up in their indexes. It's normal for Bing / Yahoo NOT to index nearly as many pages or your site as deeply as Google does. I have some sites either de-indexed or severely penalized by Google that are still very viable and profitable with enough traffic from the other major search engines to keep them live. They still make money. No sense throwing out the baby with the bathwater. Google ain't the only game in town.
3. What else is still sending traffic? Check your 'Came From / Referring Sites' in your website stat logs. Are there some strong sites, specific backlinks, or web 2.0 properties driving steady supplemental traffic? What could you do to drive even more visits from proven referring traffic sources? Using Parasite / Barnacle SEO tactics can be quite revealing. They're sites that still rank highly at the big search engines that might get your content onto Page 1 or 2 of SERPS - even when content on your own domain can't.
4. Have you left stones unturned? Say you never seriously pursued a press release marketing strategy, posting to document sites, or never opted for getting guest posts on niche related blogs: What previously unused marketing tactics could you leverage to get new referring traffic QUICKLY - and possibly for the long haul?
5. Have you got Nothing To Lose? Understand your remaining options:
A. Let the domain die a slow death until its registration expires.
B. Register and launch a new site to start with a clean slate.
C. Risk 301 re-directing the old domain to the new one and pray penalties don't follow.
D. Continue to maintain the site minimally, limiting your time involved to match lower traffic / earnings.
E. Optimize the On-Page Headers for Bing / Yahoo and be happy with that.
F. Throw everything - including the kitchen sink at it - and see what sticks.
Personally I prefer the latter. If there's a dime to be made, I'll throw a nickle at it. Finding ways to get ROI from a Google de-indexed domain is a great learning opportunity. Free from annoying Google traffic cluttering up your stats, you can often more clearly see what DOES work.
Get Revenge! Often if a site is deindexed, I'll immediately yank Google Analytics and Webmaster Tools code if it was installed. No point in continuing to feed them information about the domain THEY killed. Sometimes I install AdSense on domains they've de-indexed. I can deliberately cost them money and earn from ad clicks on sites they de-indexed regardless. In fact, I only run AdSense on sites that have been hit with Google penalties!
You might opt to TRY aggressive or what you thought were 'risky' strategies and link building automation tactics that you were previously afraid of experimenting with. As long as they can be done cheaply, with minimal time investment, you might unearth tactics that still work - at least short-term.
By and large, webmasters are DELUSIONAL about recovering from Google slaps. Google doesn't really CARE if your site exists or not. Stop thinking your domain matters one bit. It doesn't.