Hitting the debt wall is a dire financial situation that can occur when a nation

Nation

A nation may refer to a community of people who share a common language, culture, ethnicity, descent, and/or history. In this definition, a nation has no physical borders. However, it can also refer to people who share a common territory and government irrespective of their ethnic make-up...

 depends on foreign debt and/or investment to subsidize their budget and then commercial deficits stop being the recipient of foreign capital flows. The lack of foreign capital flows reduces the demand for the local currency

Currency

In economics, currency refers to a generally accepted medium of exchange. These are usually the coins and banknotes of a particular government, which comprise the physical aspects of a nation's money supply...

. The increased supply of currency coupled with an increased demand then causes a significant devaluation

Devaluation

Devaluation is a reduction in the value of a currency with respect to those goods, services or other monetary units with which that currency can be exchanged....

of the currency. This hurts the industrial base of the country since it can no longer afford to buy those imported supplies needed for production. Further, any obligations in foreign currency are now significantly more expensive to service both for the government and businesses.

This same concept has also been applied to personal debt. Specifically it has been applied to students who get in over their heads with student loans to finance their education.