If you suddenly encounter this error when try to build a Android app by using
Buildfile: build.xml does not exist! Build failed
Try this command to fix it:
android update project --target 5 --path /path/to/android/project
target is the build target for your project. Run:
android list target
to get a list of all available targets.
This error may occur when you try to go back to an old version of ADT.