By default, when you review your Widgets in Manage Widgets or your Posts in Manage Posts you see a date range of 90 days. If you click on the date range it will open up the following options:
You can change the date range to a shorter time-frame or customize to a larger time-frame.
It could be that the Widget you are looking for was prior to 90 days and that's why you didn't see it in your list. Or you have so many Widgets you couldn't see the trees in the forest!
Take a look at how to select a Custom Range: