need to reinstall widget category on nexus 7 I have tried to find answers to this problems at several different forums.  I went to the all apps screen where the widgets category tab should be and it was gone.  I even tried to down load a widgets launcher from play store and it didn't work.
Many APPs have widgets that you can install on a panel of your home screen.  Long click on a panel of your home screen that still has sufficient empty space left for your widget(s) and then click on Widget then on the widget's app you want.   If there was no space left on the home screen panel you were on, you would not be able to see widget choices to be able to install a widget.  

You wouldn't find Widgets on the App Screen unless it is an individual widget App.  There is a resemblance between apps screen and home screen, and that is what you probably remember.

You could have possibly answered your own question here, by clicking on the "Widgets" tab on your question, or better by clicking on tag at the top of the page and they typing "widget" into the tag search box,  or using the search box at the top of the page.