Schäuble pointed to conflicting reports on whether Athens still plans a referendum Sunday on bailout terms and whether it would support a yes or no vote, noting that "all of this is no basis for discussions on serious measures."
"That's why first of all Greece must clarify its position on what it wants, and then we will have to talk about it, under conditions that are now far more difficult," Schäuble told a Berlin press conference.
Schäuble pointed out that the proposal "to be rejected or accepted doesn't exist anymore and never existed" -- because the European offer in question was rejected by Athens last week and the aid programme formally expired on Tuesday night.
Since the programme ended at the stroke of midnight, "the legal and actual facts" had entirely changed, he said.
"We are ready for any eventuality, but we are in a really difficult situation," he said, blaming "entirely the behaviour of the Greek leaders, which is no longer comprehensible to anyone".
German Chancellor Angela Merkel told MPs in the Bundestag (German parliament) later on Wednesday that "the future of Europe is not at stake" from the Greek crisis but warned against striking a compromise at any price that could weaken the EU.
"Yes, these are turbulent days. And the stakes indeed are high," Merkel said, a day after debt-laden Greece crashed out of an EU aid programme and became the first advanced economy to default on an IMF payment.
„The world is watching us. But the future of Europe is not at stake. The future of Europe would be at stake if we forgot who we are and what makes us strong - a community based on rules and responsibility," she said.
"If we forgot that, the euro would fail, and with it Europe," said Merkel, who through years of eurozone turmoil has championed tough reforms and cost-cutting in return for bailout cash from the EU, European Central Bank and International Monetary Fund.
She said that after years of crisis Europe had become "more robust" and that although the present situation was difficult "it is primarily a source of agony for the people of Greece".
Merkel said Europe could now "calmly" await the outcome of a referendum called for Sunday by Greek Prime Minister Alexis Tspiras on the country's bailout terms, because the bloc was "strong".
With the second Greek bailout now expired, negotiations will have to start again from the beginning to try and find agreement for a third programme under the European Stability Mechanism (ESM).
Merkel and Schäuble's remarks came after crisis-hit Greece sought to revive collapsed talks and requested a two-year rescue deal with the EU, just hours before the European part of its international bailout expired at midnight.
An International Monetary Fund (IMF) spokesman confirmed that a payment of €1.54 billion – a "bundle" of weekly payments slated for June that the Greek government had been allowed to delay – did not arrive by the deadline, with the Greeks warning the IMF beforehand.
That makes Greece the first developed country to miss a payment to the IMF.
Eurogroup finance ministers are due to meet in Brussels at 11.30am on Wednesday to discuss the situation in Greece, after the group dismissed Tsipras' new reform proposals out of hand in a conference call on Tuesday evening.
“Under these circumstances it would be crazy to extend the bailout programme. That's why it's ending tonight,” Eurogroup president and Dutch finance minister Jeroen Dijsselbloem told the Netherlands parliament on Tuesday.
German MPs are due to hold their own debate on the Greece crisis at 1pm on Wednesday. Merkel, her deputy Sigmar Gabriel and finance minister Wolfgang Schäuble are all expected to speak.