Not much info to go on there... 
Did it ever work?
Is your phone unlocked?
Are you getting any signal bars?
Are you connected to the 48 network (or failing that O2)?
Have you got data roaming enabled?
Do you have membership?
What does your online account say about your membership status, expiry date and call/texts allowances?
Any chance you breached the fair usage limits and got barred for the remainder of your current membership month?
Hope this helps.
