Started happening to me a few hours ago....
tapping close to the power button seems to work but it shuts down once u stop.
attempted to repair it myself but im not seeing anything physically wrong with the tiny power button (inside).
leaving the phone off for a few hours. hope that works.....anyone else found a solution besides returning it to Google?
edit: I solved the problem!!!! (voided my warranty as well. lol)
the physical power button (internal) was stuck.
1. I opened the nexus 5 (using any repair guide)
2. I located the internal power button (very tiny)
3. I gave it a few taps with the base of my screwdriver (not too hard but not too soft either)
after re-assembling (partially) the phone came on for about 10 minutes but the problem started happening again so i disassembled it once more.
THE FOLLOWING MAY NOT BE AN ACTUAL FIX (I PROBABLY JUST GOT LUCKY)
4. (looking from above) the power button is square (half is white and the other half is yellow).......i used a razor blade and gently separated the two halves....not completely separated, just a tiny bit. completely separating the white from the yellow would destroy it.
5. gave it a few taps again and now the phone is working fine.
I hope this helps someone who is unable to return their phone to Google.
I tried to explain what I did as clearly as possible. I will be happy to answer any questions asked.
nivet5, did you have any further reboot loop problem? I am from Brazil and I got the sameproblem, however the NEXUS is not sold here so I would need to send it to USA to some friend and, after they replace it, he/she would send it back to me... I think it would take a long time and it can be expensive. So maybe I could do it by my self, like you did. I just would like to know if this really solved your problem before opening my device and loose my warranty.