High sierra
I downloaded an update for high sierrra yesterday, since then my computer goes round in circles - trying to start from the apple with the line gradually going across to my password page, an page telling my computer has a problem and going back to the apple and line again. I have restart, closed down my computer and started it up again and closed it down and left it over night. But the same thing is still happening. What can I do? Please help.
MacBook Air, Op system 10.13.2