If this happens first you need to question yourself. Did you something wrong? The reputation score is not going to drop like that and there has to be a reason associated with it. A person whoes score has been affected is the first judger to it - whether it was appropriate or not. Considering the scenario that a person was not involved in breaking any of the community guidelines and was ethically operating then the person needs to raise an alarm.
By raising Alaram I mean approaching or joining the discord channel to cross check with the team on the reason of scoring leading to zero. I'll share with you an instance that happened with me which is not directly tied to the score but will help in understanding to solve problems tied to it.
One of my posts was marked as Spam by steamcleaners indicating it as a plagiarised content. The reality was that I have my own personal blog and I used steempress plugin to post the same content on steemit. I, approached the discord server to prove my identity and ownership of the blog and once the team was satisfied with my justification they removed their plagiarise comment from my post. The point is that if a person is on right track and following all the community guidelines then for no reason the score is going to drop. And if for some technical glitches the score did drop then the best way is to communicate via the discord channel and put forth your case and I'm positive the things would get solved easily.
Alternately if a person tends to move in the wrong direction then you know the consequences which could result in a score to be zero or negative as the case may be.
First of all, that cannot happen without a reason. Either because I'm doing something wrong or someone is really angry with me--someone with large steempower. So that will only be a call to change. If my actions have offended someone then I will seek redress.
In terms of being flagged, there are ways to resolve the issue. There are people who can help out in case of malicious flagging. But if it is a case of plagiarism and your account has been blacklisted then there is little you can do about it, except starting a new account and changing your ways.
Ouch, that would be a nightmare.
Anyway, I have a quick answer to that ~ forget about that account and start all over again. There is no way that you can emerge out from the depths of the unknown unless a bunch of whales will come and rescue you.
If the account is just a lowly plankton or minnow and no where near any of the whales, then the best thing to do is to move on and charge it to experience.
If in case you will take my advise, it's better that you reflect on the things that you've done that cause your reputation to drop down to zero and never repeat again.
Be a good Steemian :)
You could easily make another account if your low rep were the result of abusive flags. You could power down and move your funds off-chain into your new account. That way the abusive flaggers wouldn't be able to follow you. After a while you could tip your most important and trustworthy supporters as to what your new account were in private memos.
I would just carry on to be honest - reputation score doesn't mean a whole lot to me, and can be easily gamed through buying votes. The only way it would negatively impact me is that some posts and comments would become blanked out or hidden.
I have seen some users get flagged to negative rep, and still posting away like nothing ever happened! I do think that unless you do something really wrong here then there is no reason to get flagged. In saying that there are a few people on Steem who seem to be out for a fight, and looking for any reason to flag people. In this case I just stay away from these people