I just find this story terribly sad. Really. It just seems to sum up all the bad stuff about the current situation.
Is this an EVA ticket? What airline ticketed it? Check the ticket number for the first 3 digits and find out. The airline SHOULD be responsible for re-routing you on this short of notice, but they may just try to shunt you over to Chase Travel, in which case you might see if they can find a re-route for you. But if you want to get home, you may not be able to be picky about things like aircraft type and routing. However, the booking airline needs to make good on this, either with your directly or through Chase Travel. If you REALLY need to get home, you might have to get creative and just get a refund on this ticket, and book a new itinerary. It might be necessary to do separate tickets. Find out what major hubs you CAN get to from Singapore, then see if you can book separate tickets to PDX from there. This is the sort of thing that's worth racking up debt on a credit card.
It's also possible that showing up at the airport tomorrow, an agent there might be better equipped to help you.
I hope you'll be ok and not homeless and depressed.