You probably had an warning saying 'lsa shell export version has encountered a problem.....'. If this happened you have got the sasser worm. Afterwords the pc will shutdown. If u get the pc running again at all and this happens, click start, then run, then put in 'shutdown -a'. This stops the computer from switching off.
If u cant get your computer to switch back on either bring it to a specialist or say hello to your reinstall disc.