Recently, I got lots of negative reputation. Why? I guess, I earned all those reputation by accepting answer. But, some answers were written by me. Although, I lost reputation. But, I didn't get reputation for accepting that as answer.
You need not to worry about this. Someone who has deliberately downvoted most of your posts but you need not worry. This is serial voting and your reputation will be restored to you by 5 am UTC. If the 24 hours have already passed and the suspicious votes have not been reversed, you can then flag one of your own posts for "in need of moderator intervention" and explain in detail what happened. Also you can go to the bottom and click on contact (or in case you can't find it here is the link. Just explain them if within 24 hours your rep hasn't been restored and you feel someone is deliberately doing it. You will then soon receive information in your gmail from them within 24 hours.