Almost all cell phones will reset or power off if you continue holding the power button for up to 30 seconds. Once its shuts down completely then power it back on. Or If possible you might try simply pulling the battery out , wait a few mins, replace battery and power back on.
Have you tried pulling the battery out while unplugged. I would try that, pull it out for a few mins and then plug in the power cord, no battery and see if it will turn on then. If not reinstall the battery and try again