Because one of your drive has crashed. You need to find which drive crashed and uninstall the drive and then your PC will can boot with normal mode.
Run "msconfig" go to "boot" (may be boot.ini) and unchecked safeboot, reboot your PC, now you can boot with normal mode, but I guess you may get a blue screen with some error message, when this time you can find which drive crashed, or you can upload your dump file and we can help you to find which drive crashed.