GRAND ISLE, La. — Nearly two dozen turtles that were stranded by cold weather last year in Massachusetts have successfully undergone rehab and have been returned to waters off Louisiana’s coast. More than 1,200 young, “cold-stunned” Kemp’s ridley sea turtles were stranded in November and December. According to the National Oceanic and Atmospheric Administration, “cold-stunning” occurs when the circulatory systems of sea turtles exposed to frigid water temperatures for several days slow down to a point where the turtles cannot function. Various sea-turtle rehabilitation facilities along the east and Gulf coasts joined in the effort to care for the stunned turtles. Suzanne Smith, stranding-and-rescue coordinator for the Audubon Nature Institute for Marine Mammals and Sea Turtles, said 21 of the 27 turtles she and her colleagues received were released into the Gulf of Mexico, 24 miles off the coast of Grand Isle, on Thursday. “It was a beautiful day,” she said.