First thing to try is unplug, pull the battery, and leave it out for about 15 minutes. While you're doing this, also remove the SIM. Clean the battery and phone contacts and you might as well try to clean the power connector too if you can and look in there to see if anything looks bent out of place. After you replace the battery and SIM, try turning it on first without plugging it in and then if that does not work, try with it plugged in. The one time this happened to me, I replaced the battery with one of my backup batteries and it turned on and then I swapped the original battery back in and it worked again. If you don't have a backup though, do the above. Hope it works.