Hi there,
Your status didn't expire - it doesn't expire, it must be renounced or revoked - and that's why you still have it.
Here's the problem: they now know that you are a permanent resident and have been living outside of the country for 43 years. It's pretty unlikely you will be allowed to re-enter Canada again, though it is
possible they could let you in, because you're an American. (I wouldn't risk the attempt, personally.) Basically, you can only enter Canada with the appropriate status/travel document and you don't have one. Getting your PR Card (which you can only do in Canada) or applying for a Permanent Resident Travel Document, would cause your status to be investigated and revoked.
Had you learned this while in Canada, it might have been a different story as you could have, conceivably, waited it out for a while (2 years), and got a PR Card.
To the best of my knowledge, the only choice you have now is to complete the renounce application.
Here's some information about it:
https://www.immigroup.com/news/pros-and-cons-renouncing-your-permanent-residency
http://www.cic.gc.ca/english/information/applications/voluntarily-renounce.asp
And here's the application:
http://www.cic.gc.ca/english/pdf/kits/forms/IMM5782E.pdf
Sorry I don't have better news.