Stocks started a short trading week with big losses Monday as the crisis in Greece escalated. The Nasdaq tumbled 2.4%, marking the biggest loss since a 3.1% drop on April 10, 2014. The index sliced its 50-day moving average and the 5000 level, which it had defended twice earlier this month. The S&P 500 slumped 2.1 % for its fourth straight loss. It undercut prior lows near the 2100 level and turned a touch lower for the year.