You may have encountered some errors and to prevent it you may have cleaned it by pressing Clean from Project in your Menu-bar .
You have a file in your gen folder R.java
It may go missing and you may get lots of errors in your classes .(like : R cannot be resolved to a type) . Do not try to correct it by importing R .
- Just check that there are no errors in your layouts (xml files) from res .
- There may be some problem in the style.xml or some API level problem in AndroidMainfest.xml
- Build your project by going to Project in your Menu-bar and click on Build All .
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