Hi there - Have you tried leaving it on the charger for awhile? That's usually the first thing to try. Let it get some juice in it and then try turning it on. If that doesn't work, you may need to download the WDRT program on your computer and plug your phone in there to reset it.
If you need additional help or have more questions or details to share, please join the site so you can reply in this thread. See this link for instructions on how to join Windows Central.