It depends a bit on how long it was like that. It is most likely fine. The alcohol in the beer creates an unfriendly environment for bacterial infection. Even if you did get some oxygen exposure (and you likely had a some co2 over the beer) the negative effects of oxygen exposure develope over time. Just replace the o-ring, carb it up, chill it, and see how it tastes. If it tastes good just drink it and don't let it hang around too long. It's probably fine.
If it tastes bad, well then you have to let it go. You do have the chance that a fly or fruit fly or something could have found it's way in there. In that case it's time to brew more. How much was left anyway. If it was at a wedding reception I would expect it saw some action.
Primary: German Hef, Belgian IPA, Scottish 80, Belgian Dubbel
On Tap: Oatmeal Stout, Vanilla Oatmeal Stout, Belgian Dark Strong, Munich Dunkel, Dunkel Weizen, Oktoberfest, Bock, IPA, Black IPA, English IPA, Pale Ale
Using the mind to look for reality is delusion. Using your senses to look for reality is awareness.
"One time I was so desperate for a beer I snuck into the football stadium and ate the dirt under the bleachers." Homer Simpson
Life, Liberty, and the Pursuit of Hoppiness