Question: I don't know why I cleaned the project, but I did, and now one of my activities is reporting 50 errors, like:

```
    setContentView(R.layout.activity_settings);
    // activity_settings cannot be resolved or is not a field

    CheckBox check_sound = (CheckBox)findViewById(R.id.settings_sound);
    // settings_sound cannot be resolved or is not a field

```

And so on. This was working fine before I cleaned the project and nothing has been deleted. Only one activity has errors, no other errors reported in any other file. Tried restarting, Build automatically is enabled, Project>Build and Project>Build All are greyed out.
What's going on?

Answer: 
Your R.java is missing probably in the gen folder. Right-click on the Project root --> Android Tools --> Fix Project Properties
or even deleting the gen folder and then again cleaning the project will regenerate the R.java file again.

try this too : <http://blog.burnayev.com/2009/11/android-developer-tip-regenerating.html>
hope this helps.
@timsim

1) To do so just right click on yout Android project:

* Android tools
* Clear Lint Markers
It worked perfectly for me.

2) try to check preferences->Android->SDK location. In some case, Android sdk requires higher version of ADT. Refer to this link: This Android SDK requires Android Developer Toolkit version 14.0.0 or above. .... Please update ADT to the latest version

Let me know if you face any issue.

Thanks