I know this is an old post, but it was one of the first hits for my search to figure out the answer to this exact question.
I found the answer! Basically:
1. Follow instructions here to view completed tasks.
2. Next to your grayed out item there will be a checkbox with a checkmark in it, take the checkmark out.
That's it! Your task should now be back in your "To-Do List" without a line in it.