Help phone wont register sim card samsung galaxys2

[classymums]

i turned my phone off yesterday and when i turned it back on it was stuck in flight mode, after hours of frustration i factory reset the phone thinking this will solv the problem, it did, but now it wont register the sim card. the sim works fine in another phone, when i put in sg s2 it shoes a circle up top with a line through it, and if i go to "networks" it says insert sim card to check network status.
Help please
ive only had this a month