If it is just a problem with getting the radio rom on the device there are a few threads that discuss this in detail.
Here are a couple. Just do a search for "radio" > How to: get your Radio Back!
& Radio Dead?
If it is a hardware issue you will have to send it in for service. I had this problem a few weeks back and they had to replace the main board in my 6700. The radio would just come and go. I could get the radio rom to reload after messing with it for a while and then it would just quit again. Since the repair I have not had one single problem. UTStarCom only had my 6700 for 2 weeks and did not charge me for the repair because my 6700 wasn't more than a year old.