Here is where I read about it: different problem but more detailed instructions.
[TIPS] Broken Power Button? No Problem! - xda-developerspecifically from the link above:
2. So your power button doesn't work, and your battery has died or has fallen out. How are you supposed to turn the device on without a power button?
All you have to do is plug in the phone to an AC charger, and the phone should come on.
Sometimes, however, it will show the boot logo, then go to a screen showing you your battery percentage, and not actually boot. No problem. Just pull the battery and unplug. Replace the battery, and while holding the "x" button on your keyboard, plug the phone back in (be sure to keep holding the "x" button until a triangle with an exclamation point appears). Your phone will reboot into recovery (if stock recovery, press the magnifying glass). Simply unplug the phone, and choose to reboot, and poof, your phone is on.
3. So you need to SBF, but don't know how to get into the bootloader without the power button?
Simply plug the phone into your computer. Sometimes if the phone is already off, it will automatically boot into the bootloader. However if the phone is already on, just use the ADB command "adb reboot bootloader"
If your not good with ADB, simply pull your battery, replace it, and while holding the "up arrow" on your keyboard, plug the phone in and it will reboot into the bootloader.