I am sure I am not the only one having this issue cause my friend started having this issue this week. When you start having issues with your Nvidia share feature there is a fix for that without restarting.
Well lets begin another simple fix. All we need to do is open task manager and close anything related to Geforce Experience. Remember nothing Nvidia related they are the green icons so only the black ones.
You may have to re-login into Geforce Experience but now you can go back into your game and use Nvidia Share again.