I see three possibilities.
First they can reach an agreement with the rest of the Eurogroup. I think this would be essentially equivalent to voting yes on the referendum. I don't see any reason why the other 18 in the Eurogroup will make significan concessions. Creditors have good reason to make an example of Greece. The governments of debtor countries have a much better reason to make an example of Greece -- otherwise they will lose the next election to the party most like Syriza (which in Spain is at least Podemos -- a party like Syriza -- while in Italy it is the moviment 5 stelle which is a bunch of angry people who shout a lot but don't have a program and who are responsible for the fact that Berlusconi can bring down the Renzi government and I hate ... sorry back to Greece).
I do not think this is an acceptable choice. The problem is that the rest of the Eurogroup refuses to accept Keynesian economics in spite of the overwhelming evidence. Their proposed solution to the Greek mess is not a solution at all. The problem isn't just that they are demanding even huger sacrifices from the Greeks. The problem is that their approach will not work on its own terms (see Austerity Arithmetic).
Second, they can stick with the 60 euro limit on ATM withdrawals until their banks run out of Euros. Notably this includes trying to stay healthy without pharmaceuticals.
This is clearly not an acceptable solution. It also seems likely that they will try.
Third they can reintroduce their own currency. They absolutely don't want to do this. I'm not sure why. The cost of introducing a lower status currency is that Greece would have to declare a banking holiday -- oh it has already done that.
Now if they don't like the word "drachma" they don't have to call it the new currency the drachma. I propose they introduce a new currency initially worth 50 Euro cents and call it the Deutsche Mark.
They certainly have the sovereign right to do this, and I think the threat might be useful.