Why is my score suddenly 0 in Kattis?

Kattis automatically reviews and flags submissions for potential plagiarism and other types of cheating. If an account has a significant fraction of submissions which are flagged it will be automatically removed from the ranklists. If this happens, the score for the account will be displayed as 0. 

To resolve this situation, and return the account to the ranklists, you will need to solve a significant number of problems without any flagging. The exact number depends on the number of flagged submissions on the account, and the number of submissions in total. But as a guideline, you will need to solve about 5 problems cleanly for each flagged submission.

Note that the ranklists are intended to track individual performance. Submitting the same code from multiple accounts, downloading submissions from the internet and similar activities is likely to quickly bring an account above the threshold for automatic ranklist removal.