I have had my Nexus for 4 months and I could not start my phone. The start button did just not respon. I tried the: out with the battery, plug the phone to the computer and a so called "beep" did not happen, and it turned on by it self and did not turn off at all.
This is my solution:
1. Once it's on- try to copy all your files to gmail or external hdd.
2. Go to settings.
2. System resore.
3. Done.
That is the instruction I've got from the service desk. It's not the best solution, but it works :/