- when you say no service, do you mean no data service, or even no phone service?
- does this happen when you restart?
- have you tried turning on airplane mode, then turning off?
- If you have phone service, have you tried updating the PRL (dial *228, choose option 2)?
- settings->wireless & networks - is "mobile network" turned on? If you go into mobile networks from there, is data roaming and enable always-on mobile data turned on?
Let's start with these.