The barn was set on fire by Rick and Carl once they captured the walkers in there. Everyone leaves in groups of 2 or 3. They all seem to find each other later once they get to the highway and then again at a makeshift camp. They all get together and realize that Patricia( Otis’ wife) was killed by walkers and nobody had heard from Andrea and assumed she was dead. Meanwhile Andrea was running on foot, and managed to get away from the farm with all of those walkers. There were so many walkers and it is because of the helicopter that flew over head that day. Anyways Andrea became so tired and worn out from fighting all of these walkers on her own that she just didn’t have any fight left. She was barely hanging on when walkers were again attacking her and from out of no where a person in a hooded cloak with a sword comes in from no where and kills them. It’s Michonne and she has two armless walkers on a chain leash as her protection. How cool is that?! So, she saved Andrea and they hunt and walk together. And then we see the prison in the background and that’s the end of season 2.