My HTC M8 can't start. Just keeps rebooting itself for the past 24hrs.

My phone is a HTC M8. For the past 24hrs, it's just been rebooting itself: from the HTC start screen, to Android is upgrading, then it comes on. Just when you are about taking control, the cycle starts again.

I've tried the hard reset: holding and pressing the on/off button and the volume key for 10-15 seconds.

Pretty much frustrating. I don't know if anyone here has experienced such before.

Thank you.

Bring the phone to your carrier, find out where they handle problems with phones, go there and see if they can reflash the last update. (A bad update can cause what you're describing.)

This is assuming that you didn't mess something up yourself. Flashed a 3rd party ROM? Rooted?