If the phone won't power on, make sure the battery isn't completely drained.
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.