If you are tracking your web traffic via google analytics you may come across some of the unknown sudden fluctuations in your traffic reports.
There are two cases, first case would be a sudden spike in your traffic reports and the other would be a sudden drop.
I have figured out some of the reasons that cause the drop in traffic
Has your tracking code be altered on website?
Has the Analytics Tracking Been Removed/Altered?
Any significant changes to site ?
Your website being hacked ?
Spam sources has inflated your analytics setup ?
See the original question: https://www.en.advertisercommunity.com/t5/Reports/One-Day-Traffic-Drop/td-p/601491#