Android eclipse -感叹号旁边的项目名称。

I changed the dependent projects of one of my project and moved one of my files there as an experiment. I now moved it back but the project won't run. It tells me I have an error even though I can't see one and "Clean project" doesn't help. However, the project has an exclamation mark next to it. What does that mean?I changed the dependent projects of one of my p