Hi Mamta, my name is Alvaro and I'll be glad to help you today.
I'm sorry to hear that you're having issues unblocking a contact. I understand that you already tried to unblock the said contact from your mobile. To further isolate your case, can you please try it as well through a different platform? Here's a complete article with the step-by-step process that includes all possible platforms that you could use to unblock the user.