You can only enter Canada with a PR Card as a permanent resident, officially. However, there are exceptions that you may be able to get away with:
- If you are driving into Canada, or coming in on a private (non-commercial) boat or plane, you might be able to use your record of landing
- if you do not need a visa to enter Canada, you might be able to enter with just your passport
In all these situations, you will likely be detained and questioned by CBSA before they let you into the country.
Just so you know, though, if you are returning on a commercial flight (or a cruise ship) you will likely not be allowed to board without your PR Card (if you need a visa to come to Canada).
You should look into getting a Permanent Resident Travel Document if you must travel without a PR Card: http://www.immigroup.com/canadian-permanent-resident-travel-document