We have a bathroom on the second floor that we never use. Two days ago we noticed that water was leaking out from under the carpet and the bathroom sink was filled with water and not draining. The AC condensation drain runs down this drain right above the trap. We took the trap off and snaked the drain but the water will not drain at all other than to fill the trap. While the trap and the small condensation drain pipe had some pink color slime, there didn't seem to be enough buildup for the drain to seem so clogged. I was talking to a co-worker and he suggest that there might be something wrong with the vent on the roof - perhaps a dead animal or debri buildup in the vent. Could this be the problem? I've read that you should either snake the vent or take a hose and run water down in it - which is recommended and if you snake, how long a snake should be used to snake the vent? Also I also read that if you run water down the vent, there is a possibility that the water may gush out of the toilet or sink. Some thoughts on how to best approach this problem would be appreciated.
Thanks.

"We took the trap off and snaked the drain but the water will not drain at all other than to fill the trap."

If you have snaked the horizontal drain line in the wall and still have standing water in the lavatory then the blockage's down stream from the sanitaary tee in the stack vent. The solution would be to snake the lavatory roof vent. Put out enough cable to reach the base and 30 foot more. Good luck, Tom