Use Telegram’s global search. Tap the Search bar (magnifying glass icon) at the top of the main screen.
If you blocked someone and then deleted the entire chat, finding them might seem tricky. Here’s how: how to unblock people on telegram
If you have deleted the chat history with the blocked person and don't want to dig through settings, you can search for them directly. Use Telegram’s global search
You can also tap the Edit button at the top-right of the Blocked Users list to remove multiple people at once. How to Unblock Someone on Telegram Desktop and Web how to unblock people on telegram
Tap on their name at the top of the screen to open their profile.