This is a known issue as long as you just updated instead of uninstalling and reinstalling you should be fine. Just press alt+z to bring up the share menu. However if that doesn't work you will have to wait as it appears their servers are running very slow it took a few hours for it to tell me that I could use share however mine was fine throughout so I had no issue.
Just keep opening up geforce experience every half hour or so and checking the settings page and share should eventually pop up.