My mac says you computer was restarted because of a problem

Hello, I recently faced a problem that when I started my mac to open, it showed the login screen and as soon as i entered password I was displayed by the screen saying your computer was restarted because of problem then repeatedly this message was shown....

My mac is not opening now.... Whenever i press power button but now not even login screen is presented... Help me out. Its just showing that pop up notification your computet was restarted because of a problem repeatedly..