That is a tough spot to be in for sure, but hang in there. First thing's first - drop your rate and make sure you are the lowest in your area for awhile. Since you're in Portland, you'll have more competition, but that also means you will have more people looking for services, and many will overlook one bad review if the rate is cheap enough. A lower rate will be temporary until you rebuild your reputation. Once you lower your rate, advertise like crazy! Advertise (and RENEW your post frequently) on Craigslist if you're comfortable, post to your city's Facebook page, print flyers to hang at local businesses and bulletin boards or put under car windshields (make sure that's allowed in your area), create a Facebook page and Google page (free) just to have more website presence, talk to local veterinarians or stop every person walking a dog that you see, compliment them on their dog and give them a flyer or business card (if you have them). When you do get another stay, make sure it goes amazingly well so the owners will feel compelled to leave you another glowing review - take lots of great pictures every day with detailed, positive updates, and send a little goody bag home with them at the end. (You can get a lot of ideas for goody bags on these Rover boards.)
You can also sign up for other dog sitting sites, though Rover so far is my favorite, and it does get challenging managing requests from multiple places.
Hope this helps, and good luck!