Once my friend submitted my source code, now my ratings are dropped. In May challenge in solved 3 problems still I got <1000 rating. What should I do now to get rating as others.
You cannot gain all the ratings at once. Participate in the rated contests and solve at least one problem. I believe you will start gaining ratings like others after 3-4 more rated contests. In June we have 4 rated contests left. Participate in all of them and your rating will be above 1050 (if got sufficient rank) at the end of June.
Codechef’s rating mechanism is complex to keep all aspects in mind that can affect your ratings including old performance vs current performance, and plagiarism which you just faced. Because of plagiarism decreased your ratings to 0, rating mechanism will also consider this… In simple words keep participating in contests … You will be on level soon… All the best!
You are facing this issue because you have been plagiarised in multiple contests. Ratings of those affected by plagiarism in a single contest are now back on track if they participated in 3-4 contests after that. For multiple plagiarisms, it will take a longer time.