Over the years I have had my fair share of this type of frustration myself. One problem I found was that my electrical ground fault was tripping when the pump in the tank started. Years ago the electrical code required that the electrical connections to the septic pump be on a ground fault circuit. This requirement was changed because of the very problems I just alluded to. A second problem I have experienced a few times is the lack of a weep hole in the discharge line. In both cases the discharge line gets water in it and freezes the line. It is critical that the discharge line drain back so it does not freeze. Please feel free to call for more information of this topic as it is not easy to cover all the possibilities in a small space as this. The picture is a poor example but it gives you the idea. The weep hole sometimes gets plugged and the water cannot drain back into the tank. In a pump out the water must not stay in the  discharge line as it is above the frost line and it will freeze. In an emergency you could place a trash pump in the lid of the tank and pump the effluent above ground to where it is normally pumped out to get you by for a few hours until you can get the line thawed. This will require a very long hose. I am not suggesting you do this for more then a day. You need to put ethanol in the pump out line which may clear the line. Secondly you need to check to see if the weep hole is clear. Finally hire a vac truck to drain the tank if you can’t free up the line within a few hours of discovering a full tank. You will then need to get a steamer to melt the ice. Once you get the line clear it is imperative to make sure the weep hole is spraying out water, thus draining back when the pump shuts down. septic tank pump out