Are you sure that the battery has a charge? You might want to try recharging the battery for a while and try the reboot again. If that doesn't work, you can try restoring the device.
Look at this now - none of the other steps worked, so don't be put off by that sentence.
- Connect the device to your computer and open iTunes.
- If the device appears in iTunes, select and click Restore on the Summary pane. Learn more about restoring iOS software.
- If the device doesn't appear in iTunes, try to force the device into recovery mode
If you want to read what all of the other steps were in that support article - read this.
Frozen or unresponsive iPad
Resolve these most common issues:
• Display remains black or blank
• Touch screen not responding
• Application unexpectedly closes or freezes
iPad Frozen? How to Force Quit an App, Reset or Restart Your iPad
What to Do When Your iPad Won't Turn On
iOS: Not responding or does not turn on
iPad: Basic troubleshooting