There are two pathways to try to re-enter Canada if your PR card has expired.
The first is to fly to the US and then try to re-enter Canada through a land border using a private vehicle. If you are allowed in without being reported for failing to meet the residency requirement, then you would then need to live in Canada for 2 straight years without leaving to meet the residency requirement. Once you meet the residency requirement, then you can apply to renew your PR card. Note that while you living in Canada and waiting to meet the residency requirement, you won't be able to sponsor any family members for PR and it may be difficult to access certain services such as the provincial health care system. If you are instead reported when you enter Canada for failing to meet the residency requirement, you will have to appear at a hearing to argue why you should be allowed to keep your PR status despite note meeting the residency requirement. Unless you have very strong H&C reasons, you can expect to be refused and will then formally lose PR and be told to leave Canada.
The second option is to apply for a PR Travel Document from outside of Canada with H&C considerations. You want to have very strong H&C reasons for having failed to meet the residency requirement. Reasons such as working or studying outside of Canada won't be counted. If the PRTD is approved under H&C, you can then travel to Canada and apply to renew your PR card as soon as you arrive. If the PRTD is refused, this will trigger the process to officially revoke your PR status.
Those are your options.