Why does build hang at Building Resource Folder?

My game was building perfectly fine up until a little while ago, when it became stuck at "Building Resource Folder". I tried the simple things I could think of, but I don't know what Unity is trying to do at that step, so therefore I don't know why its getting stuck and how to fix it.

Are there any common practices for when this happens? Is there anyway I can track down what went wrong?

Thanks, Dale