No sim card installed on my iPad2?, No sim card installed on my iPad2?

I just cant use the 3G on my iPad 2... it keeps saying sim card not installed... it was working fine few weeks ago and i was even able to control if i wanted the mobile data to be turned off or turned on.... but now it says SIM CARD NOT INSTALLED and the mobile data option in the setting is greyed out.... i cant switch between mobile data to turn on or turn out...... plz help really frustrating....