A few minutes ago S&P downgraded Greece to BB+ from BBB-. That looks like a minor step down but it's actually a move from "investment grade" to "junk". The Greek government has admitted they've stopped trying to borrow on the bond markets - the yields the lenders are asking are prohibitive. They can't go to the ECB anymore, their bonds aren't acceptable.
So all that's left is the IMF. An IMF team is in Athens at the moment but no deal terms have been made public. In practice the German Chancellor is calling the shots. Greece wants 40bn euros and that would come from Germany mainly.
Even if Greece stops borrowing now they still have to rollover their existing debt. There's a 9bn euro bond due to be redeemed on the 19th of May. If they haven't the cash by then it's all over - they're bankrupt.