|Did it boot properly earlier?|
if it did then, try to power it up and enter F8 it will take you to
the boot menu from which you can select the option " LAST
KNOWN GOOD CONFIGURATION"
This will boot up with the last successful boot settings.
- If you installed some thing before the failure, then try to boot
through the safe mode from the same boot menu from F8 and
try to uninstall the software/driver you installed that could be
the problem too.