Method 1: One way is to use the Undo command in the Edit menu, but that may not be the right solution if you sorted some time ago and don't want to lose the changes you have made since then.
Method 2: Another way is to delete the case table in which the sorted data appear and make a new case table. This works because sorting only applies to data as you see them in the case table, not to the cases themselves as they are stored in the collection.
Method 3: Define an attribute whose formula is caseIndex. When you sort by this attribute, you get the original order of the cases in the collection.