Generally sovereign debt is not a problem until creditors become worries that the debt will not be paid back. The exact threshold is subject to the regular market fear/greed manias and is not an exact figure. Once this happens the country issuing credit will have trouble lending at favorable rates and will be forced to choose between printing money (could lead to massive inflation) and making large cuts. Here is recent example: https://en.wikipedia.org/wiki/Greek_government-debt_crisis and here is an older more infamous one: https://en.wikipedia.org/wiki/Hyperinflation_in_the_Weimar_R....
The sudden nature of these consequences without many real world impacts beforehand also happens at an individual level. It might be tempting to take on credit card debt at the moment and there might not be any real world consequences to running up the balance but if painful measures are not taken to address the situation before a moment of crisis (i.e. inability to service the debt), you are in for a world of pain.