It depends on the circumstance, did you screw them over on purpose? Was it out of spite? Even if it was, people change and you can't hold things against them forever.
I deleted my best friend from my life due to peer pressure and now I wish I had never done that because I want to be friends with her again. It all sounds very childish, I'm sure.
Less childish than you'd think. I know I'm just an internet random, but at the very least try apologizing to them. People find it hard to admit their mistakes and if you show your friend that you can they might find it easier to forgive you.
The thing is, I wouldn't even know where to begin! But thanks for your help, and sorry that I was talking stupidly and using dumb words yesterday. : )