Restarting so much time.

First, try a PRAM reset. To do this, shut down the computer, then power it on while holding P + R + Option + Command on the keyboard until you hear two distinct chimes. If this doesn't help, then try to boot into Safe Mode. To do so, shut down the computer, then power it on while holding the Shift key. Continue to hold the key until you see the progress bar. Then release the key and see if you are able to boot up and login. If so, login and try to apply any available OS updates. If this doesn't help, you may have a deeper issue and may need some type of hardware repair.