Hi, good day!
You mentioned that this started after a Windows update. Do you know which update was installed? You can check under Settings > Windows Update > Update history and share the KB number
Also, when was the last time the Shutdown option worked properly? Was it available immediately before that update was installed?
As an initial step, please press Windows key + X and check Shut down or sign out. Do you see Shut down there?
I look forward to receiving your update and I'm willing to collaborate with you till we resolve the issue. Do keep me informed by clicking "Add comment" below.