Well, hold down the power button as long as possible to do a cold boot,
start up the system, tap F8 just before it reaches the Windows boot screen,
on the list of startup options select 'Last Known Good Configuration'. If
that does not work, go back to the list of startup options and boot into
Safe Mode, and do a System Restore to an earlier point in time before you
started experiencing this problem.
If non of those don't work, you need to contact the manufacturer of the
laptop and get it checked out if its still under warranty.