It was revealed that Greece had a lot more public debt than they had previously reported. Their bond yields then skyrocketed to the point where it would be insane to finance government spending by selling them, because the interest payments would be so high.
Normally a country has a central bank that acts as a lender of last resort and makes sure that the government's bonds are always bought at a reasonable rate. If no one else wants to buy Greece's bonds, their central bank should have stepped in and bought them itself – all developed countries with their own currency do this.
But Greek is in the Euro and doesn't have its own central bank, it has to rely on the ECB. The ECB refused to perform this service.
Eventually, the ECB started doing this for other countries, but after the damage is already done.
To put a real cherry on top, the ECB ruined Greece possibly irreparably by refusing to buy its bonds. But now, the ECB just announced plans to "boost the economy" by buying .. wait for it … not just bonds from Eurozone government but … CORPORATE BONDS FROM PRIVATE COMPANIES.
So 5 years ago the ECB would not do its duty as a central bank and help fund Greece at a reasonable rate (ALL central banks do this) – but now, it will directly fund private corporations.