So it was working fine for a while but suddenly it's saying "no SIM"?
Even after a reboot?
What model of iPhone is it?
Was there any software update or settings changes preceding this problem?
Does reseating the SIM make any difference just in case it has somehow become dislodged?
Can you try the SIM in another unlocked phone to see if it works ok there?
Could something have changed with the phone that it needs to go through the silly/annoying Apple iPhone activation process again?
That involves the phone silently sending an SMS message to a UK activation server and this will silently fail if there isn't cash credit on your account to pay for the international text message.
Might also be worth reviewing this Apple help article:
If you need to contact 48 support about this then see the links in my signature below.
