Dell Inspiron 3520 gray screen on startup

Hi,

Latly I've porchesd Dell Inspiron 3520 with Windows 8 X64 and for a couple of weeks it's workd fine,but yestarday when I boot it up a constant gray screen was displayd until I restarted it forcely (long click on turn off  and turn on afterwards).

Since then,every time when I starting my laptop a gray sceen is display for a long few sec' and windows 8 starting afterwards.

what should I do in order to fix it?

Thanks,

momo22