plug in to charger. hold volume down key and power for 10-15 seconds, then let go. it should go into charging mode. if not, do it again. make sure you count out the seconds, or use a watch/clock. 12345 is not the same as 1 . 2 . 3 . 4 . 5 ...
if you can get it to turn off, and stay off, while not plugged in, remove the sim and sd card, and try to power on normally. you won't get service, but you'll know if the sim or sd is not working. if powers on ok, power off and reinsert sim, then power on normally. etc etc.
if you can't get it to power on normally w/o the sim or sd, then you'll need to reset the phone from the recovery menu ... from power off, hold volume up AND down AND power key. you'll get to a triage screen (looks like a dead Droid kinda). use volume down to highlight recovery, then press volume up to select it. if you go past recovery, keep pressing volume down to cycle thru... then use volume keys to highlight factory reset, and power key to select it.