Plumbing Line In Backyard?

I believe you are describing a , clean out, for your sewer main. Sounds like it is frozen.Here where I live, New Mexico, we use hot water or steam jetters to clear frozen sewers. If the line does not thaw out on its own, call a plumber with a jetter. The problem with leaving faucets running to protect from freezing is that it can also freeze the sewer line. Good luck! Hope to help!...............................