I have searched similar cases on Google, and I made a conclusion,
In total, there are three options that you can follow:
option 1:use password reset disk if you have made one. this is an easy way to reset just follow the instructions.
option 2:use command prompt to reset Windows 8 password, but you should login your Windows as administrator
option 3:use third party program to reset lost windows 8 password.Windows Passwrod Recovery Tool is a good choice that you can try, I found many users think highly of this program in CNET. You can get this program in Cnet or google it directly.