You will need a valid PR card or a PRTD to fly back to Canada. Depending on where you are travelling it can take quite a long time to get a PRTD. You also have the option of flying into the US and crossing by land in a private car with an expired PR card or COPR. All this requires you to be compliant with your RO. If you meet the RO you could file to renew your PR card now and hopefully you'll have your new PR card before you travel.