Expert advice needed on oxidation

So, last weekend at a party, we unfortunately ran out of Co2. After I had gone to bed, my roommate decided (bless him) to gravity feed a keg to get the beer out. Now, originally my main complaint is that it meant drunk people were lifting up a keg every time they needed a beer. I'm told it happened no more than 10 times, but I'm wondering...

Ideally your Co2 lines wouldn't have any leaks in them but you never know. If the Co2 lines would create a vacuum with nothing to replace the beer coming out of the keg. This was not the case, so I'm assuming oxygen got in it.

The keg sat for about 12-16 hours without Co2 in this state. What effects would it have on my beer? When I tasted the beer after reconnecting it seemed to taste watery and with no character. Carbonation is fine, but the beer doesn't taste like I remember it.

Thoughts?

It's more likely that the residual CO2 was coming out of solution and equilibrating into the headspace providing enough pressure to let the beer pour out. I don't think it saw any oxygen unless someone was venting the keg. I think what you're experiencing is a lower carbonation level than what you had previously.