Where are you located. 30 years is not that old for septic.

So when you say distribution box you are talking about in the field where the line spreads out over the whole field?

If you run water in your house you should see flow at the same rate as the water in your house after ten minutes. If you don't see flow you could have a pump system that only flows after your tank gets full.

Most families use around 100 gallons a day so your septic tank should be full by now unless there is a huge hole in it.

It is very unusual for a septic system to freeze as the biological affluent is nearly as good as glycol anti freeze. However, if the tank is above ground level, which would be very unusual too, or if you have low biological process activity and you live in a very cold climate, it is certainly possible. I suspect that the fact that you pumped the tank in December likely reduced or even eliminated the biological process activity which then caused your freezing issue.

Is there a pump in your system or is it entirely a gravity system? If it is pumped system, a hydrosplitter manifold (i.e., a manifold with plastic discs in each port for each gravity fed drain field line having a small hole appropriately sized for the elevation of each drain field line) is usually used and you can’t visibly see any affluent going to the drain field lines. Float switches in the tank are used to provide a predetermined dosage to the drain field lines and the hydrosplitter manifold ensures each drain field line gets an equal dose during each pump ON cycle. If it is entirely a gravity system, there is usually an inspection box close to the entry point of the drain field lines and you should see affluent. It is entirely possible that if your tank and lines froze, this caused damage to the tank and/or lines such that you now have a diversion leak prior to the inspection box that needs to be repaired.

It is always best to have septic tanks pumped out in the Spring to ensure that there is good biological activity before the Winter cold weather. After a septic tank is pumped out, the tank is then usually filled with water.