Charge it for a few hours and go from there. Sounds like the battery is dead.
If you hold the power button does it continually vibrate in spurts? If so, then it's most likely just a dead battery.
Here's some more help for you.
Hold down power + volume down...the phone will turn off, but don't release them yet. It will flash LG logo for a split second and then turn off completely and stay off. You may release the buttons now.
Press and hold power + volume down. when you see the LG logo release the power button ONLY and then quickly press it and hold again until you boot into recovery.
Hit power twice to enter recovery
Holding volume up + plugging in your USB to the computer will get you to download mode as well and let you stock the phone.