Sewer lines tend to freeze when they are shallow, sagged, with not enough slope and with a minimum water flow in the pipe, this usually happens when homeowners returns from their nice winter vacation and they have a leaky fixture, the small amount of water freezes slowly until the ice creates a solid blockage.
if you suspect that this could be your situation, contact instadrain inc. We can steam your sewer line, remove the blockage and get you flowing again.