A person can't tell when they've been blocked. There is an application that some people have that tells them when they've been blocked/unfriended, but not many people have it so I wouldn't worry about that.
To unblock someone, go to your news feed, scroll all the way down (it may keep loading posts, but keep scrolling. It will stop eventually), and click edit options (to the right of "load more posts"). It will show you all the people who you have hidden from your news feed, and if you click the x by their name it will unhide them.