After update from 4.3.2 to 5.0.1 there is NO SERVICE.

I'm using an Iphone 4, White, Model MC604C. I bought this from my friend in Canada when she came back to Vietnam. At that time, the Iphone's version was 4.3.2, baseband 03.10.01 and I can use it with ANY SIM in Vietnam. So I guess this is a WORLD version. But today, I decided to update to version 5.0.1 but Itunes required a VALID sim to activate. Since I've updated many Iphones from Hongkong or Singapore, it always automatically activated if it was a WORLD version. So why did my phone need a valid sim then? I tried to put in and out my Sim card from Vietnam but there is always SEARCHING... and finally NO SERVICE.

Please help me with this issue. I don't know which carrier this Iphone belongs to and my friend doesn't remember either. So I cannot ask her to buy a Sim card to activate.

My Serial is: 88118TSHDZZ

Thanks.