Make her laugh, tell her jokes if that doesnt work you go out and get her something nice. They say a key to a girls heart is an unexpected gift at an unexpected time.
Usually you guys are terrible at relationship advice, but these are actually some good suggestions! Except for bacardirum and anions. I would say: 1. Apologize first 1a. Give her a hug 2. Make her smile 3. Bring her a small, but thoughtful gift 4. Bring her a bigger gift... and the more wrong you were, the bigger this gift should be!
You've gotta do it the right way. Here: Head down, eyes looking up, hands in pockets, shoulders up, head slightly tilted, conveying shy vulnerability..: "I'm sorry baby, I love you...."
Leave her alone and let her cool off... Anything more specific would require a better description of what happened