CO2 will naturally disperse over time and you will lose your protective layer (the scientific reason is to drive down gradients in partial pressure), but I am in agreement with everyone else and think it unlikely that oxidation is the problem. You didn’t happen to have the carboy in the sunlight, did you? Or under a florescent lamp? Perhaps the beer was skunked?
That may be an even easier way to ruin a beer: leave it in a carboy in the sunlight. Wouldn’t take very long at all, especially if it was a hoppy beer.