I one had (made) an extension that had a bad manifest file in it that caused Photoshop to grey the extensions menu item.
Try removing recently installed extensions from the extensions folder and see if you can identify the trouble maker. It's probably the most recently installed extension.
After you identify it you can work on fixing it.
Sometimes restarting the computer can also fix things.
Solve my issue
What I did was go into Preference > plug-ins
Tick Load Extensions Panel
This must turn itself off during update.
Turns out to be the extension panel tick box had turned itself off on Preferences >Plugins must be a feature to the upgrade.
Jon L Miller