How To Unblock Someone On Facebook Messenger On Your Computer System? from Emma's blog

Start by clicking down arrow from the top right of Facebook, then you will reach out to the ‘Settings & Privacy’ section. Here, you have to click on the ‘Settings’ option and then select the ‘Blocking’ option. Now, you should click on the ‘Unblock’ option given next to the person’s name. Lastly, you have to click on the ‘Confirm’ option to the operation says how to unblock someone on Facebook messenger.