The problem will not be to enter Canada, but rather, to board commercial carriers. If you somehow make it to the port of entry, and establish that you are a permanent resident, then you may enter Canada by rights. This can be accomplished by presenting the landing paper and your passport.
Check the following link for more information, in particular, Section 11 (Examining permanent residents at ports of entry)
www.cic.gc.ca/english/resources/manuals/enf/enf04-eng.pdf
However, I'm assuming that you're not from a visa-exempt country? If that's the case, without the PR card, you will need to cross the border by land, since carriers won't let you board the plane that will take you to Canada.
So you will need to go to border towns in the US (such as Detroit, Buffalo, Niagara Falls, etc), then drive (or walk) your way back to Canada. Good luck.