I brewed a Russian Imperial Stout right before Christmas. I let it ferment at about 68 degrees for almost three weeks. At that point I had to leave town on an extended business trip. My wife did not want the fermenter full of beer hanging around in the closet while I was gone so I moved the whole bucket out into the garage. I left the blow off tube in place.
The end of the blow off tube was in a plastice jug that had maybe two pints of water in it. No sanitizing agents...just plain water. I was talking to her on the phone the other day and she told me that there was no water in the jug. As best I can gather what must ave happened is that the cooler temps in the garage caused the air in the fermenter to contract causing a bit of vacuum. This vacuum then sucked the water back into the fermenter through the blow off tube.
What are the chances that my beer is ruined? I had my wife immediately put water back into the jug but I am afraid the damage may be done.
You're probably ok. There's always a chance of contamination but I wouldn't worry about it too much. Taste it and see what you think, if it tastes good drink it, if not let it sit a while and see if age changes anything.
I had something similar happen: it was hot enough that the water in the bucket evaporated and the tube was clipped to the side (leaving the tube open to the air). I totally freaked out that my beer would be ruined, but it turned out just fine.