Just some checks you might want to do:
1) Check to see if you have 3G turn off in setting
2) When did you notice this happening? Was there an app you downloaded that might affect your 3G? Like it has permission to enable/disable 3G...
3) Are you able to use the net with your wifi? no=net problem in general; yes=solely 3G.
4) Try your SIM on another phone to see if 3G works. I've read that an older SIM may affect your 2G/3G performance. Does that work? no=SIM issue, yes=phone issue...Also, you can try someone's else SIM on your phone and see if you get 3G signal.
5) If those don't work... You might need to do a factory reset (back up your apps and everything before)
6) If after factory reset, you're still not getting 3G, then you'd need to contact your carrier (issue with network) or HTC (issue with phone)
Hopefully you get this fixed without going through #6