I have a better solution that worked for me and it might work for you. No need to delete the whole .idea folder as this way you lose the whole project. It is possible that you just accidentally excluded all folders in project settings. I also found that project folders disappeared. I was trying to find out how to disable indexing, so I was looking at Project Settings -> Modules and I clicked on Excluded to see if I could add the whole project there to ignore it for indexing. It didn't do anything and I couldn't drag and drop the folder under Excluded. What I didn't realize is that simply clicking "Excluded", IntelliJ automatically moves the folder to Excluded and all folders disappeared in Project Explorer. "Excluded" button changes background to gray, which means exclusion is active. This is a very poor user interface because it is hardly noticeable and clicking this button has a far-reaching impact on your project.

Fix:

打开项目设置->模块 如果“排除”突出显示，单击它 单击应用或关闭

结果: 项目中的文件夹再次显示出来

非常糟糕的UI设计!源和排除按钮看起来像标签时，他们的工作实际上是“复选框”!

注意，仅仅单击Sources或Excluded似乎是一个没有任何可见效果的选项卡选择，但它具有深远的影响，基本上是从构建过程中添加或删除文件。

我不得不退出Intellij并删除。idea文件夹(我先把它藏起来，以防万一)。然后我重新打开了这个项目，这对我来说很有效。 不要忘记在此之前保存你的配置(例如调试/运行配置)，因为它们也会被删除。

我已经删除了。idea文件夹。关闭ItelliJ并重新启动以打开相同的项目。

它要求添加内容根。这对我很管用。

你不必完全退出Intellij来完成这个工作。只需关闭已打开的项目。(您可以打开任何其他项目，以避免完全关闭) 转到已安装项目的目录并删除“。想法”文件夹中。 重新打开项目。

这应该能解决问题。它在2018年5月对我有效。

将。idea文件夹备份为zip文件 删除。idea文件夹 重新打开项目 关闭项目 将备份文件解压到新生成的.idea文件夹中 重新打开项目 赞美太阳☀️\[T]/

删除.idea/workspace.xml对我有用。关闭项目，删除.idea/workspace.xml，然后重新打开项目。我使用的是2019.1版本。