Windows 8 64 bit has running about five months on my pc and everything works fine until today...

when i start up my pc, it was at the please wait screen for about 15 minutes. so i have to shutdwon it by holding the power button at least 20 seconds. after the restart, it still stay on the please wait screen. i have tried to run other mode but did not work.

does anyone know what's wrong with my pc?

any suggestions would be highly appreciated.