Unless the driver’s side arm has stripped on its pivot, it sounds like one arm of the wiper linkage has become disconnected. To verify, you will need to remove the wiper arm(s) to check for stripping/wear and then remove the cowl to access the wiper motor and its linkage. Often, the plastic grommets or clips that attach the individual linkage pieces to each other wear out, the linkage separates, and you lose movement in one or both wipers….. usually in the rain or snow (because that’s when you really need your wipers!), and usuale thats the side you really need!).
 
Tom Dayton
JS Auto Supply
Jamestown, N.Y.