Cellular data usage?

Friends
I am using IPAD 2, with iOS 5.0.1, after updating it OTA, when i tried to use 3G on it, it started asking me for "CELLULAR DATA PLAN USAGE There is no data remaining on your current plan. Would you like to add more data now? If you don't add now, you can do it later in settings." if i click on NOW button, it doesn't do anything, and my IPAD is carrier unlocked with unlimited data plan, i don't know what to do, i have tried removing sim and inserting it again, but no positive results, then i put the same sim in my iPhone and it started working, so i think there is something in iPAD, because before 5.0.1 it was working flawlessly.
Please help me
Thanks