i am using kali linux in virtual box by oracle . I have used many times with given username and given password during installation .and vitual machine required update and i did not update that time and suddenly I could not log in kali linux . But after update and install of vm still i did not log in .when i entered username and password and press enter it again come back to log in . And after that when i tried each and everything as suggested in forums such as username:as root and pass :-as toor it says your password is incorrect .Please try again. same thing occur when i did username as root and password as given during installation . i do not understand what is the problem. Please help me out .I will be very thankful .