To put most HTC phones in safe mode, you need to boot them with the volume (-) held through the entire boot process. I'd try rebooting again/pulling the battery (if you have a removable battery).
If that fails, have a look and see if there is anything stuck or the volume button is jammed at all?
If no, can you turn the volume up when the phone is on?
If none of the above work, you can try a factory reset, if that fails, you might need to see about getting the handset repaired.