Power button not working. Phone wont boot?
Have you tried plugging in the charger? or else plugging in the charger and while plugged in, remove and then re-insert battery. Some phones tend to start up in that manner. 

Also if you can connect via adb (even if your phone is powered down), the adb devices followed by adb reboot ought to start your phone. If you're rooted, there are apps like Quick Reboot that allow you to reboot to Recovery or Download mode with a tap, provided you have CWM.