What exactly happened in Greece that made their economy plummet?

They have been in debt for decades even before changing their currency to Euros. There are many aspects with regards to Greece's downfall and it's inability to bounce back. Primarily, they are suffering from high rates of inflation and unemployment when compared to other european countries. Moreover, their constant need of borrowing money makes them vulnerable. 

During the 2008 Global Financial Crisis, Greece debt-to-GDP ratio was around 179%  and its budget deficit to GDP is around 12%  almost four times the Eurozone's allowable budget deficit. Major Credit Rating Agencies lowered the sovereign rating of Greece to "junk bonds", scaring off investors and driving up the cost of credit.  With these scenario's, Greece were having difficulty repaying its debts, thus triggering the debt crisis in Greece.

la seule et la grande cause du grèces est les banques ou bien les credits de consommation qui ont produie par les banques

That due to many reasons but the main reasons as economists agreed are 

1-Inefficient Pension System

2-High government employees benefits

3-High Unemployment and Work Culture Issues

4-Early Retirement

5-Tax Evasion

debt and too high inflation

Significant financial losses for eurozone countries and the IMF, which are owned the majority of Greece 's roughly euro 300 bilion national debt.Adverse impact on the IMF and the credibility of its austerity strategy .Loss of Greek acces to global capital markets and the collapse of its banking system.

False economic results 

Absence of transparency

Misguided targets

There was a decrease in tax collection by the government of Greece which led to increase in borrowing so as to finance the country's budget. Greece borrowed more and it became an indebted Country. Decrease in tax was caused by tax evasion by business people and corruption in the Country. This discouraged foreign investors in Greece.