Did it work before?
If not then follow the steps here to make sure that you have configured things correctly:
http://community.48months.ie/t5/Need-Help/Samsung-Galaxy-phones-internet-settings/m-p/13363#M4013
(Don't worry about the link name - it's not specific to Samsung Galaxy phones).
If it did work before or these steps don't help then log a support request to get 48 support to look into it for you. To speed things up it may help to state what geographic location you are in when the problem occurs.
I'm assuming that you have a valid membership and a data allowance.
Hope this helps.
