My mother in law has cold fever, throws up a lot can't even drink water and has diharea. what could be the cause?

Gastroenteritis: This is a very common malady and usually resolves in 5-7 days. If, however, she fails to get better or is becoming dehydrated because of lack of intake, or really has fever in excess of 100 f she should be seen by a doctor. This could be one of numerous causes, viral, bacterial, food-borne, primary bowel disease, etc. Needs a full evaluation if not clearing quickly.