My parents have been divorced for 20 years and I feel like I can even talk to my own father anymore. It's been years since I've seen him last.
Why would you feel like he couldn't talk to your father after the divorce?
Years ago some words were said face to face that we can't take back, you could say it was a heated disagreement. Now it is too awkward to try to talk anymore.
A divorce doesn't mean you don't see your kids, he should have been there for you even if it was awkward. Talk to him one day out of the blue, I am sure he would be happy to hear from you. You don't want the silence to be what makes a silly disgreement control you.
Ok, convincing me not to be upset by talking to my dad just empowered me to write him a letter.
Writing him a letter is a good first step. Your father should be there for you, and you making an effort to speak to him could help repair the relationship.