Iphone keeps dropping cellular data settings

[mr f]

Hi all I have a 2g 16gig Iphone jailbroke running on 3.0.When I go to check my email or go on-line im told cannot connect as the phone is not connected to a cellular data network, so I go into settings, general, network, cellular data network all the fields are blank so I add my apn username password go back to the home screen try to check my emails then im back to square one blank again has anyone come across this how do i fix the problem thanks in advance.

[iBwizzle]

Do you have a proper iPhone data plan?