Didn't have time to properly respond earlier, but here's a few things you can check (this is often a difficult issue to fix):
1. Did you copy code from another project into the one that isn't building? If so, you might have an "import R...." type of reference up top near you other imports that you need to remove.
2. Did you change your package name? If so, did you remember to also change the package name reference in the AndroidManifest.xml file?
3. Have you tried (for the Eclipse IDE) right clicking on your project name, selecting Android Tools -> Fix Project Properties.
That's all I can think of at the moment...pay attention to the lines that are marked in error and the mouse-over context/information it provides. Also, look in your Error Log tab for more details about what your IDE thinks is wrong.
Best of luck!