This is common question of many users on facebook that I accidentally blocked someone, can I Unblock them?
Here I am showing you that how to Unblock someone on facebook..!!!
To unblock someone on facebook:
1) log into your account
2) go to your privacy settings by clicking on the link in the top right part of your home page
3) find the name of the person you want to unblock in your "block list"
4) click "Unblock" next to the person's name
and then it is done.
If you have any problem about this or want any suggestion/help then please ask via comments.