Additional Study Information: The purpose of this study is to find out of the combination of the drugs abemaciclib and letrozole is better or worse than usual approach for your type of cancer. The usual approach is defined as care most people get for endometrial cancer. People who are not in a study are usually treated with either surgery, radiation, hormonal therapy or with chemotherapy. Sometimes, combinations of these are used and your doctor can explain which may be best for you. These treatments can reduce symptoms and may stop the tumor from growing for several months or more. Another purpose of the study is to test any good and bad effects of the drug abrmaciclib when used in combination with letrozole. Abemaciclib and letrozole could shrink your cancer but they could also cause side effects and your cancer could grow while on study treatment. Abemaciclib plus letrozole has already been FDA-approved to treat other cancers.