I updated my Nexus 5 to Lollipop a month back and it was working fine. Suddenly one day my device didn't boot up so I did a factory reset and I got wipe/cache error and if I switch off the power and restart I get the flying color dots page and nothing comes up after that. I went to service center and they said it's a software issue and they can't do anything about it. How can I fix this?
There is nothing to worry. You just have to flash the images properly.
Please follow the steps mentioned in this Android Enthusiasts answer.
The initial bootup usually takes time around 10 to 15 minutes. I'm up and running 5.0.1 without any problems.