IPAD3 3G NO SERVICE

I have bought the new ipad(ipad3) this summer and all the thing were going well. i could use my sim card to go online any where any time. everything changed last week, I suddenly couldn't connect to internet by the sim card anymore, it keeps showing " no service ", i have tried the way i could think of, and i have made sure that my sim card is ok and good

restart ipad3

restore ipad3

reset internet setting

try another sim card

everything just happened suddenly, what should i do? i don't wanna give up my 3G function!