If your sewer is backing up only when you flush a toilet, run water down a sink drain, do laundry, or wash dishes; the plug would be in the house plumbing or the sewer service line that connects to the City’s main. In this case, you should call a plumber or rent a machine to unplug the line.
On the other hand, you can recognize a plugged City main because sewage will continually backup and not stop until the City responds by unplugging the City main. Contact the Police Department at (308) 762-4955 to report a plugged sewer and the on call person can assist in determining the location of a plugged line.