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.
This happens a lot, at least for me. Eclipse is kinda frustrating at times. 

This could be something as simple as a typo error in a layout file or a string file and it crops up after a project clean.

Unfortunately eclipse doesn't help and throws only an APT error rather than just showing where the error is. Rest of the times it behaves well by showing errors.

May be you could double check all your layout files and string files thoroughly. One way could be trying to see a graphic layout of your layout files, if all is well, a graphic preview is shown.