若干年前我曾 landed 左加拿大, 但沒有留下來坐移民監, 返回香港繼續生活, 現在想再移居加拿大, 請問要重新申請嗎?

The answer is mostly no.

Assuming your PR Card (formerly known as Maple Leaf Card) has expired, the easiest is to apply for a travel document:

http://www.cic.gc.ca/english/information/applications/travel.asp

If your application is a success, you are still a PR. Otherwise, you are not.

Note - Canadian immigration law is going through a series of changes. It will no longer as easy as before to keep a residence without living. So you may be good for now, but it won't be for the future.