To Unblock a member
Go to Profile & Settings > Privacy > Blocked. There you will see a list of all the members that you have blocked. Press the 'Unblock' button next to the member you wish to unblock
To Unhide a member
Go to Profile & Settings > Hidden Members. There you will see a list of all the members that you have hidden. Press the 'Unhide' button next to the member that you wish to unhide.
You can also unblock, or unhide a member from their profile page.