If you cannot add them, they have probably blocked you. So before you take action, consider if they want to be friends with you again - it might save both of you from awkward (digital) situations.
If you are willing to get in touch via FB again, you could send them a message asking to unblcok and/or add you.
Your message might end up in the "Other" folder, which people often don't read. So if there is no response, you could ask some of your mutual friends to ask your former friend to look into their "Other" folder.
Again, you probably blocked each other for a reason, so consider carefully if getting in touch again is wise.