The most frequent issue is a dodgy USB cable - they can be surprisingly fragile but are at least cheap and easy to replace (around $1 on ebay). To confirm this is the issue, try another (borrowed?) USB cable.
If the USB is OK, then it's most probably drivers, you need to:
- disconnect the phone from the laptop
- uninstall all phone drivers and all phone-related software from the laptop (e.g. Kies/HTC Sync)
- restart the laptop
- restart the phone
If your phone comes with PC software - e.g. HTC Sync or Kies - re-install this however, do NOT connect the phone until prompted.
If your phone does not come with PC software, connect the phone to the laptop.
NB: in BOTH cases, do NOT disconnect the phone from the laptop until the drivers have all installed and the connection is working.
