Can't sign out, restart or shut down

Hi, just over a month ago I brought a new Windows 8 Laptop (Toshiba C850) and all was going well - up until about two weeks ago when I started having all sorts of problems... Basically, I can't log out, restart or shut down my laptop anymore. Obviously this is a real nuisance, and I've been having to manually shut down the laptop by holding the power off button every time, which can't be...