Recently bought Nexus 5 and yesterday while talking it got off due to battery getting low . At this time i put the same to charge with my laptop and after half an hour also the same could be switched on , today while contacting the LG service centre they confirmed that it would only turn on after put ro charge for at least 2 hrs. and it is not like one of the other phones which may be immidiately turned on when put to recharge .
I've never had that problem. The few times my phone has died from a dead battery it turns on pretty fast after putting it on the charger.