I hit my neighbor's new car yesterday, what to do?

My neighbors (a married couple in their late 40's) are out on vacation for 2 weeks. Here is what happened yesterday night:

After assisting a party with friends, we went to a bar/club afterwards. It was after 3:30 am when I started heading back home.

All was fine except I hit their car. Though it just felt like a minor hit (well I thought) so got home, took a shower and went to bed.

I woke up late with some headache (hangover basically) and stepped out to check the minor hit. To my shock, the headlights are broken and the bumper Is nearly ripped off. Oh shit. My car was a bit of damage too but theirs is more visible.

So what I'm gonna do now?

I hit my neighbor's new car yesterday, what to do?
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