Your PR did not expire, your card did. Your PR status is lost once there is a formal determination that you failed to meet your residency requirements.
Working for the UN is not one of the exemptions to the residency obligation. Should you attempt to return to Canada, you run the risk of being reported and having your status revoked.
To return to Canada by commercial transport, you will need a Permanent Residency card, or a Permanent Resident Travel Document. Applying for either of these may result in your Permanent Residency being revoked.
Some people travel to Canada by pretending to be visitors (visa free) to the airline, then identifying themselves as Permanent Residents to Customs and Immigration once they land at the airport. Starting in March of next year, visa free travel will require an electronic authorization, which will close this option for Permanent Residents.
If you want to maintain permanent resident status, then you need to meet your obligation. Failure to meet your obligation puts your status at risk.
