Did it work before or are you just joining 48?
If you are new then have you activated your SIM?
If you have and are using an iPhone (looks like it from your post?) then there may be an additional hurdle due to the stupid way that Apple do things.
In some cases and on some networks an iPhone needs activation.
This process involves the iPhone sending a UK SMS behind the scenes.
On 48 you need at least 25c cash credit on your account to cover the cost of this SMS.
And if you don't have it the iPhone doesn't tell you what's wrong.
So if you are new to 48, have activated your SIM, your iPhone is unlocked for use on any network and it is still not working then try putting a small amount of cash credit on your account, switch off and on again and wait a while and see if it eventually starts working.
As I say if iPhone activation is the issue here then blame Apple for the stupid way that they do things in this and other contexts (e.g. FaceTime activation, iMessage activation, roaming problems, iMessage problems etc.).
They really make it difficult for users to use their phones on some networks unfortunately.
And their FAQ on the "no SIM" message is next to useless:
48's FAQ might be a bit more useful:
Hope this helps.