I sure hope not! That would be considered a disaster when losing 90% of traffic on 5000 visitors per day. That leaves you with 450 visitors which if your income is based soley on Google traffic means layoffs and possible closure.
I would have been on the horn contacting Google immediately if anything like that were to happen. Either you've been manually banned or, you have a "major" technical glitch or, I don't think there is another or.
What is blackhat stuff?
I'd say that would be a possibility if there were other signals present but we don't know the whole story so we can only provide clues.
Contact Google, file a Reinclusion Request. Login to your Google Account and ping them immediately. Provide a complete outline of what has happened and the steps you've taken to address any potential guideline violations if there were any.
Has anyone been fiddling with the technical underpinnings of that site?