If the phone is not powering on, first make sure that the phone doesn't have a completely drained battery:
Plug the
phone into the charger for at least 5-10 minutes to allow the
battery to charge.
Make sure you're using an
original HTC-branded charger and cable, and check the wall socket to make
sure the charger is receiving power. It's best to charge using a wall socket instead of a USB connection when troubleshooting power issues.
After you have confirmed the phone is charged, try to power it on by pressing and holding the POWER button for 10-15 seconds.
If the phone still doesn't power on, holding down certain hardware buttons can force the phone to reboot. See How do I reboot the phone using hardware buttons? for details.