Cyprus is small enough not to be really significant in economic terms for the Eurozone as a whole. (Let's treat the strategic significance of Cyprus giving exclusive gas exploration rights to Gazprom or a Mediterranean naval port to Russia in return for a bail-out as a separate issue for now).
So the mostly German bankers who run the European Central Bank (ECB) decide this is a good opportunity to demonstrate to all and sundry the consequences of not playing ball with your creditors. The ECB refuses liquidity to insolvent Cypriot banks because the Cypriot Government refuses to implement the ECB bank bail-out plan. The banks have to limit withdrawals (say to 20k Max) because they cannot sustain the outflow of funds that would otherwise occur. The banks are declared insolvent and a liquidator is appointed. Bondholders/depositors are given shares in the banks in lieu of their investments over 20K and the banks re-open under new "ownership and management".
Russian "investors" (otherwise known a money launderers) lose their shirts. Some bigger businesses become illiquid and may have difficulty trading. Small businesses and individuals are generally ok with 20K working capital but don't trust the banks as a place to maintain their working capital funds. A mattress/cash economy emerges. This works fine for small local cash businesses but business trading with other Eurozone countries cannot get credit and may have difficulty settling accounts with other Eurozone suppliers/customers. Cyprus may have to issue its own currency again but has difficulty getting it accepted by its trading partners. The larger economy grinds to a halt. The tourist industry almost collapses. Cyprus is effectively quarantined and "thirdworldised". German Finance Minister Schäuble has already said on ZDF TV: "The business model of Cyprus is no longer sustainable. Someone has to tell the Cypriots."
ECB central bankers look on with some satisfaction because this will teach the Greeks, Italians, Spaniards, Portuguese and Irish et al just what will happen if they don't play by the rules. They are happy enough the Russians got burned, but may cut a deal with Putin to stop the aforementioned Gazprom or Naval Port scenarios. NATO threatens a blockade if Russia does try to set up a naval base in Cyprus. Turkey decides this is good time to consolidate it's position in Northern Cyprus by formally occupying and developing Varosha in Famagusta. Europe is once again on the brink of war.
All of this may seem ridiculous when you consider how small Cyprus is in the larger scheme of things. And then you remember that WW1 started because of the murder of a relatively obscure Archduke in Sarajevo. Once again the greed of central European elites threatens the security and prosperity of the continent as a whole. Those who refuse to learn the lessons of history are condemned to repeat it.