Mac will not start after update

I updated my Mac today. The machine is a stationary older model. I do not remember how old, but between 5 and 10 years. After the update was completed the machine automatically restarted. But when it was starting up I saw the apple logo and the line building up. But when the line is 25% finished it starts all over again and again and again…Somethimes the screen goes black and I hear the start up sound. But the the line come back again. How can I get out of this loop? How can I start the machine?