In addition to Ofnuts advice
You probably deleted or renamed the path to the default scripts folder in Edit -> Preferences -> Folders -> Scripts and Plugins
You should have something like this:
An indication of that is all the guides and sota-chrome-logo are script-fu and 'live' in
C:\Program Files\GIMP 2\share\gimp\2.0\scripts if that is scrambled then no scripts.
There is a reset button to get you back to defaults.
Python-Fu is a different matter, you need plug-in (s) that use that as menu location.
This example: Only one in my Gimp profile folder and I made the menu entry for menu bar.
Check what you have in your gimp profile C:\Users\your-name\.gimp-2.8\plug-ins. Are the missing plugins in there?
Check the path to the plugins in Preferences. Try the reset button.
edit: Are you using Rod's method and putting various plug-ins in their own folders and adding those in the Preferences. You might have already done a reset and lost them.