Try using below code:
//Display name of your field
var fAssignedTo = "Assigned To";
var _PeoplePicker = $("div[title='" + ppTitle + "']");
var peoplePickerEditor = ...
Did you, by chance, try to open it using the Visio Designer? (DO NOT DO THIS FOR COMPLEX WORKFLOWS) I have done this for a complex workflow and it broke the Text-Based Designer. I'd also look to reinstall SharePoint Designer, as reinstalling fixed this for me.
Please check this other post here, they suggest installed Service Pack 1 to clear the issue.