Why won't my Google Nexus 5 work?

Hi, yesterday I started an update on my google nexus 5 but once it had finished updating it wouldn't turn on again properly. If I press the power button the screen saying google will come up then it goes onto the screen with the 4 coloured dots but then they just spin continuously for hours. Ive tried everything including a factory reset but nothings working.
Can anyone help?
Thanks!