Nexus 5 stuck on “flying color dots lollipop” screen after Hard Factory Reset

Hi i have been using nexus for the past one year .I updated my device to lollipop a month back and it was working fine . Suddenly one day , My device didn't boot up so idid factory reset and I got wipe/cache error and if i switch off the power ans restart i get the fkying color dots page and nothing comes up after that .I went to service center and they said its a software issue and they can't do anything about that .Need help on resolving this issue