Maybe it isn`t a version problem. Check for console messages. Probably there is some mistake (syntax error, maybe) with your .xml files (like Strings.xml, for example). Also verify if there is any resource with one or more characters in uppercase. Are examples of non-valid resources names: Image.png, opa_Draw.png, myImage.jpg. Your resources identifiers must be all in lowercase.
Check also if the project building is being made automatically.
IF YES, the R class must appear again (if it does not appear, there is still any error in the project. The R class can not be generated based on wrong information.)
IF NO, build the project manually, and then the R class appears again.
Please, try it out and tell me if it worked.