If it's not rooted then the only ways that the system software can be corrupted are via a bad update (e.g. interrupting it part way through) or a problem with the storage.
If you can find a HTC ROM Upgrade Utility (RUU) for your device, download it to a PC, put the phone in fastboot mode, connect to the PC and run the RUU (which is a Windows executable) that will completely reflash the phone. The RUU does however must have newer (or the same) software version as your current version, and must be for the same region/carrier as your phone was made for, or else it won't run. So we'd need more information to help find one for you.
If you can unlock the bootloader (via HTCdev.com) then flashing TWRP shouldn't be hard (use fastboot to flash it - you need that to unlock the bootloader, so the unlocking instructions will tell you how to install it). You do have to use a TWRP built for that exact model. I'm typing this on my phone on the way to work so can't provide links or search for resources at the moment.
If your phone is still under warranty that would be a third option.