Here are some things to try:
Navigate to the project at the top of your hierarchy, make sure Child Projects is selected at the bottom of the project navigator, and look at the full list of Features on the Portfolio Items screen. Include the Project field in your list to see where your feature is sitting - it may have been moved.
If you don't see your feature, check to make sure you don't have any filters set.
Check the Recycle Bin in the lower right-hand corner to see if it was inadvertently deleted.