can anyone help me

today i was trying to install macOS sierra on my imac. i wondered off and let the update install for a bit. and when i come back i try to turn it on all it gives me is a screen that says that my imac had an issue and was starting up again. so it gets to the loading screen and its bot loading. it stays there and tells me to press a key on the keyboard but it just doesnt seem to work. ive tried everything but its not working.