In extremely cold weather, small amounts of water (like a dripping faucet) can freeze layer by layer inside a cold pipe until it is completely blocked.
If you can access the cleanout, this is the safest chemical method. sewer line frozen
If any part of your sewer pipe is exposed (common in crawl spaces or mobile homes), check for visible frost, condensation, or physical bulging. In extremely cold weather, small amounts of water
If you catch the problem early, you might avoid a total backup. Look for these "red flags": In extremely cold weather