Q: Iphone 4 not starting up

First, try to reset the phone by holding the sleep and home button for about 10sec, until the Apple logo comes back again. If this does not work, try to connect in recovery mode, explained in this article:

iOS: Unable to update or restore

I had similar issue with my iPhone 4. I tried the steps in the troubleshooting doc but, in vain, did not help. But, one of the options I tried is to press the power button and home button together for 10 sec or more when plugged in to the power and to my surprise, it started ..!!  not sure what went wrong but, this worked .. :-) I suggest to try the troubleshooting steps first and if those don't work, try my option.. good luck..!

Thanks Prabhakarkamath, it works.

Thanks for your help, that helped me a lot!!!!!