I resided in Canada for 5 years, during these period I applied for my PR card and received it around 2008. I left Canada a year after I received it and my PR card expired in 2012. Is there any way I can renew it now? If not, do I have to apply again to enter Canada?
You are still a PR. PR status does not expire.
PR cards expire (like a passport expires, which does not terminate one's citizenship).
Your long absence from Canada means (unless you qualify for an exception, which appears highly unlikely) that you are in breach of the PR Residency Obligation. Thus, unless you have compelling H&C reasons, any application you make for a PR Travel Document will likely be denied.
Without a valid PR card or a PR TD, it is unlikely you would be allowed to board a flight to Canada.
You are, nonetheless, still a PR. So you are not eligible for some other kind of visa or for eTA (for travelers with a visa-exempt passport). The fact you are almost certainly an inadmissible PR does not alter this.
To apply for a visa, including to apply again for a PR visa, you will need to first renounce or lose your PR status. So, you can renounce your status or you can apply for a PR TD, and 60 days after that application is denied, you will no longer be a PR. Once you are no longer a PR, you are again a Foreign National and you can apply for any visa or status in Canada that you are otherwise eligible for. Fact that you renounced or lost PR status attendant the failure to comply with the PR RO should have little or no negative influence.
As Buletruck observed, if you have status to travel to the U.S., you could travel to the U.S. and then to Canada by private transportation, to cross at a land crossing. You would most likely be reported for the breach of the PR RO, which you could appeal or allow that process to terminate your PR status (the PoE issued Departure Order, resulting from being reported, will become enforceable, terminating PR status, in 30 days unless you appeal). You could also renounce PR status at the PoE (if you can get to one) and apply for visitor status in order to enter Canada.
Some PRs in breach of the PR RO have compelling H&C reasons which can enable them to retain PR status despite a long absence from Canada. It would require especially compelling, extraordinary and exceptional circumstances, to successfully make a H&C case with how much you have been absent from Canada.