Try pressing the left Control, right Alt and the Delete keys
simultaneously. What happens? You should get either a shutdown or
shutdown requester or the Task Manager. If the latter, try shutting down
the system through it. Does any of that work? If not, then, you may
have to press the Reset or Power buttons to turn the system off, then
reboot and hope what's causing the shutdown problem has fixed itself. If
not, a Repair Install may be in order. I would certainly do a very
thorough malware scan in any case.
Stef