I've had that happen a couple of times. Usually a reboot of the phone & PC fixes things. If not, try deleting the sync relationship between the phone & PC by:
- Go to Settings, Applications, Manage Applications
- Delete the data in Calendar Storage and Contact Storage
- Delete the data in PC Sync
- Reboot the phone
- Open HTC Sync on the PC and go to Tools, Phone Monitor Options, and delete your phone.
- When you switch to HTC Sync on your phone you'll be prompted on your PC to add your phone
Hopefully this will work for you