Technically, unless you got notified otherwise, your green card is valid and you can use it for entry (source).
Lawful permanent residents (LPRs), refugees, and asylees will continue to be able to use their Non-citizens Registration Card (Form I-551), issued by DHS, or other valid evidence of permanent residence status or refugee or asylee status to apply for entry to the United States.
After a prolonged absence you may run into issues with the CBP. Absence of 1 year or more requires special handling by the CBP which may end up in denying entry or referring you to an immigration judge for revoking your status, and which could be avoided with presenting a re-entry permit (if the absence is for up to 2 years), or SB-1 visa.
You may end up being notified that the CBP officer determined you've not maintained your residence in the US and deny entry, at which point it would no longer become valid (the process is a bit more involved than that, but that's the basic summary). Given your fact pattern, this outcome is likely. You'll need to show a good reason for absence, a good reason for not asking for a re-entry permit before leaving, show your maintained ties to the US (like tax returns, bank accounts, property, etc), and be prepared to go to the immigration judge with all that before being deported.
SB-1, if given, will shield you from all that, but you'll need to provide all that information and details to the consulate when applying for SB-1, and the likelihood of getting it depends on how convincing you are that you really had to leave so soon and for so long and you still see yourself as a US person. Which, IMHO, is not very likely.
You may want to talk to a US immigration attorney to discuss your situation before you do anything.