have an Asus Notebook X501A and it has just happened to me; I am running Windows 8.1 and I have done nothing at all that may have caused this.
Here's what I did that was successful:
1/ Select the Boot menu in the Aptio setup utility, select 'Launch CSM' and change to enable
2/ Select the Security menu n the Aptio setup utility, select 'secure Boot Control' and change to disable
3/ Select Save & Exit and press 'yes'
4/ Hold down the power button until the unit switches power off completely
5/ Switch back on and continually press the F9 button for about ten seconds; this should now allow the unit to boot up into the Windows Recovery menu
6/ Select 'Advanced Start Up' and wait until the recovery menu appears
7/ Select ' Advanced Options'
8/ Select 'System Restore' and select a date prior to when the problem occurred
System Restore worked perfectly for me; good luck