When I turned on my Mac it went from a grey apple and a white background to a grey blocked sign, the white background and a loading symbol underneath

My Mac was being really slow it froze so I restarted it. It started of normally with a white background and a grey apple, next time I look at it I noticed that instead of an apple now it's sort of a blocked sign I'm not to sure what its called it looks like the one people put on non smoking areas, but it still has a white background and it also has a loading symbol and its loading (obviously). It has been like this for the past hour or so what should I do?