safari disappear upon mail account set up iPhone

I have problems with my iphone 4 as my safari disappeared suddenly. I restored my phone as i tried resetting my home screen and enabling restrictions. However when i set up my mail account, the safari disappeared again. Hence i realized that its due to the mail account setup. Previously i have not experienced this problem as my iphone has safari and mail accounts setup. Now my friends also have this problem (even with 3GS) hence i hope someone could help us to fix this. Any suggestions will be greatly appreciated!
Iphone 4 iOS 4