Hi @JoanneRocque ,
If you mean that you changed the "assign to" area to just a single student to give them an extension, you can just click the "+add" button below which should then default to "everyone else". You can fill in the proper dates for the everyone else selection, then save and all of the submissions and grades should return.
The How do I assign an assignment to an individual student guide has some more info and screenshots about this entire process for your reference.
Hope this helps!
-Chris
