You must live in Canada for 2 years in each 5 year period in order to fulfill the residency requirements. There are very few exceptions to that rule. If you were living with a Canadian citizen spouse for example or you were transferred overseas by a Canadian employer. If you have Humane and Compassionate reasons for not being able to meet the requirements, you can also try and see if they will allow you to keep your PR. Such reasons could be taking care of a seriously ill close relative for all those years and possibly they would allow it because of your own health problems assuming that Canada health care could not give you the medical care that you needed.
