The most common reason for connectivity issues is a dodgy USB cable - try a different one.
If that's not it, then it could be MTP was unselected and needs to be turned back on. In vanilla Android, this is Settings .. Storage .. Menu .. USB computer connection - it's probably slightly different with HTC Sense.
If that's not it, then it will be drivers:
- disconnect the phone from the computer
- uninstall ALL phone related sofware (inc HTC Sync) and drivers from the computer
- restart the computer
- restart the phone
If you decide to re-install HTC Sync (you can probably manage without it), do NOT connect the phone until prompted
Whether you install HTC Sync or not, after first connecting the phone via USB, do NOT disconnect until ALL drivers have successfully installed
Alternatively, you could look at something like AndroidSync which works over WiFi