My M7 won't reboot past HTC splash screen - gets to that point then switches back off. What can I do?

[AC Question]

My M7 won't reboot past HTC splash screen - gets to that point then switches back off.

Phone won't switch back on! Unfortunately volume up button not working so limted options re soft boot etc! Has been a great phone for 3 years!

I've tried rebooting under light source etc.

Can anyone help?