As you may know, in order to maintain your permanent residency, you must meet certain residency obligations to maintain your status as a permanent resident.
To meet these residency obligations, you must be physically present in Canada for at least 730 days (2 years) in every 5-year period.
This means that you can spend a total of up to 3 years outside of Canada during a 5-year period.
Basically what happens is that, each time you enter Canada, Citizenship and Immigration may calculate 5 years back from the date you have entered or re-entered Canada to see if you have fulfilled your residency obligation.
You can find information on the process that is followed when entering Canada in this Citizenship and Immigration Canada (CIC) ENF 23 - Loss of Permanent Resident Status manual.
Here is an excerpt from the manual on what happens at the port of entry here:
It is important to note that in terms of loss of permanent residency, a person does not lose it until a final determination has been made.
The onus is on each individual permanent resident to make sure they are meeting their Permanent Resident requirements.
If you have family members in Canada, you may want to have them call the Citizenship and Immigration Canada (CIC) Call Centre directly for some information on what your options are and what the process will possibly be when you are returning to Canada.
I hope this information is helpful. Please let us know if you have further questions and if there is any follow up to your question/situation.