My school cancelled a LOT of things due to COVID-19 in 2020-21, what shoul I do?

So sorry that so many of your ECs and competitions were cancelled. First of all, you can absolutely alert Harvard admissions officers to this in your application. The Common App has an "additional information" section tailor-made for this kind of thing. This section even contains a "Covid question," which is a space dedicated to giving you a chance to explain the effects the pandemic had on you personally and academically. You can also have your school counselor explain this situation when they submit your transcript and other supplementary materials to Harvard and your other US schools.

It is definitely possible to get into Harvard without competitions, provided you have an excellent GPA, a good SAT or ACT test score (if you're planning to send them), and great extracurriculars and essays. Of course, it's best to stay active in terms of extracurriculars if possible. To that effect, it seems @McKellarr has given you a great list of potential activities below. For more advice, check out our blog post on this topic here: https://blog.collegevine.com/extracurriculars-you-can-do-at-home/.

You're in the same boat as basically every single other student who will be applying to college in the coming years. Colleges will be expecting dips in grades, activities, test scores, etc. for the year 2020. You don't need to specifically inform the admissions officers because they most likely already realize it.

You can still be accepted into a competitive school but you also need to realize who you are competing against. You will be going against lots of students who had lots of things cancelled too but managed to find ways to still make up for it. @McKellarr gave you a great list of things to consider moving forward and make sure you continue to do your ECs that weren't cancelled. Keep up your grades as well and best of luck with your application!