Transmission problem, help!?

Ok well my wifes car is a 2006 scion xa. It has 80000 miles on and the transmission fluid had never been replaced. It says that it doesn't need to be under normal frolic conditions, but I just found out Toyota recommends changing it every 60k. Anyhow yesterday wife turn on the car and press on gas but would not move. Checked transmission fluid and it was empty. I added some and it's fine now, at least I think so, If it's leaking it's a slow one. My question Is where did the fluid go? I though transmission fluid couldn't be burned? What could be the problem? Do we need a new transmission or is it something smaller? In 2 weeks we were going to drivecross country is this a good idea now? Please help, thanks.