If you left in 2004, you would have stopped meeting the residency requirements in 2007 and would have lost your PR then if immigration was aware of it. However, if they never officially revoked your PR, you still have it. If you manage to get into Canada without being reported for not meeting the residency requirements, you could live in Canada for 2 years straight and thereby reinstate your PR status. If you apply for a PR card after the 2 years, they will give you one because the criteria is that you meet the requirements in the 5 year period before you apply.
As for how to get to Canada, if you are visa exempt, you show up at the border and try your luck. If you are not visa exempt, if you can get to the US, you can show up at the border and try your luck. If you can not get to the US, you would have to apply for a travel document to enter Canada in which case you will have to deal with the Canadian embassy about that and it is unlikely that they will issue you a travel document for being away so long. That is unless you have spent the past 8 years taking care of your seriously ill relative or something similar.
