you R.java file will be automatically deleted on cleaning the project if any xml file in the project having error so first correct your errors in strings.xml file then clean the project . you must have changed.
<string name="app_name">your app name</string>
app_name here . if you have done so then again make it as it was or go in manifest file
<application>
...
android:label="@string/app_name"
</application>
also change here the name what you given in strings.xml file. hope you understands ...