All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The parents have a great status, and Allaah gave them great rights on their children, like being kind and dutiful to them, keeping ties with them and obeying them in what is permissible. Allaah Says (what means): {And we have enjoined on man to be kind and dutiful to his parents.}[Quran 46:15].
Cutting relations with the parent or misbehaving towards them is being undutiful to them, and this is a great major sin. In this regard, please refer to Fataawa 84723 and 89078.
Therefore, you are obliged to sincerely repent for cutting relations with your father, and perform good deeds and acts of obedience as much as you can so that Allaah may forgive you this great sin. For the conditions of sincere repentance, please refer to Fatwa 87903. However, you should not despair of Allaah's Mercy, as this is committing another sin, rather you should expect good from Him.
If your father died as a Muslim, you should supplicate Allaah as much as possible to forgive him and have mercy on him, and perform other actions by which one way be kind and dutiful to his parents.
For more benefit on how to be dink and dutiful to the parents after their death, please refer to Fatwa 83233.
Allaah Knows best.