1
Right click your Start Button, is there a power off option on that menu?
If not, on your Desktop, press Alt + F4, does that bring up the shut-down dialog and are you able to shut your PC down form there?
2
Did you install the Respondus LockDown Browser on your PC to take an exam, that is the usual cause of the power options being missing.
3
Click your Start Button, type regedit and hit Enter to open the Registry Editor.
Click View and make sure 'Address Bar' is turned on.
Paste this into the Address Bar at the top and hit Enter.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\Explorer
Please provide a screenshot of that Registry Editor page.