As long as you didn’t complete the entire resource…
on the daily plan, where the resource is scheduled, you will see at the top the last assignment with the date completed (or that it was omitted) – if you cursor over it, there will be a link that says “edit assignment”. Click on that, and it will become the active assignment and you can change it from being completed or omitted now.
(I’ve even done that a few times – to go back a few assignments…)