I think Heather was going the right direction, and Maniac brought up the point that more information would help.
Where are you blocking this number, from your contacts, from a messaging app, where?
Is it a number you have in your contacts? What kind of contact is it, Google, phone/SIM or something else?
Did you do this on your phone or online with your carrier?
If it's a contact stored with your Google account opening your contacts, sometimes under the heading "People", scrolling to the contact and expanding it then scrolling down to disable or enable as the case may be should restore call features. Messaging apps are going to have their own protocol but some may not allow you to simply turn contacts on and off. There can sometimes be a wait, for instance you may have to wait 48 to 72 hours before you can reverse an enable or disable. How long ago did you disable the number?
More info will help.