This is the part that surprises most people. Unblocking does not retroactively bring back the lost emails. Those messages sent while the address was blocked are gone forever (unless the sender resends them).
Once you unblock, future emails will arrive normally. To be extra safe, you should also add that address to your or create a Filter that says "Never send it to Spam." gmail unblock email
The Great Gmail Escape: How to Find and Unblock an Email Address You Didn’t Mean to Block This is the part that surprises most people