How to unroot nexus5 and install OS in nexus5 which i was deleted accidentally

hi

this is vinoth, i have tried to root my mobile by seeing nexus5 rooting from you tube and i thought i have sucessfully done rooting, but i was wrong bcz when i installed titanium backup and super su app in my mobile and when i start run these app i came to know it was not sucessfully rooted, after few months i got a android update from 5.0.1 to 5.1. after downloaded it asked for restart to install suddenly it took to TWRP and i selected a option bootlader and it started booting contineously and when i again start to boot it in a recovery mode it just says there is no OS installed in ur mobile please give me some solution to resolve this problem