http://www.canadainternational.gc.ca/hong_kong/visas/tdpr_info_tvrp.aspx?view=d
Losing your permanent resident status
Your permanent resident status allows you to live in Canada, but there is a time limit on how long you can live outside the country. There are several ways you could lose your permanent resident status:
•If you did not meet the residency obligations you could lose your permanent resident status. To keep your status as a permanent resident, you must live in Canada for at least two years within a five-year period.
•If you are convicted of a serious crime, you may be deported from Canada and lose your status.
•When you become a Canadian citizen, you are no longer a permanent resident.
Note: If you obtained permanent resident status in the past, but have not lived in Canada for many years, you are still considered to have Canadian permanent resident status unless you have previously lost the status through a determination made by an adjudicator at an inquiry. If you want to travel to Canada on a temporary basis (i.e. to visit, work, or study) you must have a determination of your residency status performed before you travel to Canada.
If you want a formal determination of your residency status you must submit an application for a permanent resident travel document.
For further information on losing your permanent resident status and ways to meet your residency obligations, please see the Citizenship and Immigration (CIC) website.