cant login

So,a bizzare bug happened a bit ago,still having trouble fixing it. My friend can't log into her pc. It accepts the password,it goes to black screen for a bit,then boots her back to the login screen again. Restarting doesn't work. Can someone please help,im not really sure why is it happening...

Okay so bought a Dell inspiron 7th gen i5 laptop 2 months ago, so its outside the warranty dates, and I would appreciate some input. Every time I open my laptop after it has been closed and gone to sleep, the login prompt disappears. The login screen is there, and the mouse and keyboard work...