FRANKFURT—The euro-zone economy stalled in the second quarter, raising the ugly prospect that the region's meager recovery has lost momentum just as it faces fresh headwinds from Russia and Ukraine.

Germany's economy, long Europe's growth engine, shrank for the first time in more than a year, a development economists largely attributed to a mild winter that boosted activity in the first quarter at the expense of the second. The bigger concerns, they say, are France and Italy, where respectable rates of growth aren't even...