I have a Nexus 5 that is approximately 4 months old. One time after the phone was off I went to charge the phone via a wall outlet. The phone then displayed it was updating the firmware on the phone. After multiple hours the phone had not surpassed 0% (I though it might be charging over wifi that I was connected to prior, but it seems that it was trying to download via USB). So I unplugged the phone and was able to turn it off. On turning the phone back on, it started a boot loop that has not stopped. I tried to load the phone through the recovery mode however that has not helped. When I researched the "boot loop" topic it seems to occur when mods, roms or custom OS have been loaded. However I have never loaded these softwares so that is not my problem.