It’s the app with a music note inside. You’ll usually find it on the home screen or in the app drawer (if you have an Android). This has a globe or magnifying glass icon. A list of results will appear. If you’re blocked, their account will hide its bio and videos, and you will see "You can't view this person's videos due to their user privacy settings".  This may not mean you are blocked, however—some accounts are hidden to everyone except a few people. If you are able to follow this person (or able to request to follow), you haven’t been blocked. If you see a message that says ″You can't follow this account due to their user privacy settings,″ you’ve likely been blocked by this user.

TL;DR in Vietnamese:
Mở TikTok. Mở trang khám phá. Nhập tên người dùng của đối phương và nhấn phím Search (Tìm kiếm). Nhấn vào tên người dùng. Nhấn Follow (Theo dõi).