you need tyo restore your phone to factory settings reinstall apple iTunes on a new sim with tmoile or usa at&t sim then after you've synced the iphone with the computer and iTunes replace your old sim and it will work fine heres the method step by step - happens a lot with jailbroke phones
The reason this happens is just so you know , the os system has been updated and your jaikbroke phone isn't being regognized with the right settings , this happened after a apple update a year or two ago . The actual logic behind the solution is simple . You must find a sim that preferably is a contract which will have data plan options you don't need to buy the data plan of them your just borrowing it so it can regognize your old sim that suddenly stopped working . use a friends sim aslong as its contract remember it has to e the same network the phone was originally on which I find mostly is t mobile in uk or at&t in the us .
Factory reset your iphone then with old sim not regonized it should say activate or a message along those lines . insert the borrowed contract sim and your usb to your iphone and laptop making sure you have a up to date iTunes installation on your laptop it will start the process on its own once plugged in just wait a good while may need to replace wire a few times but it will start to sync with the borrowed sim .
After synced and all finished your phone will be working again with the borrowed sim now add your old sim it can now recognize the operating system and network provided settings allowing your old sim to work again resync through usb and then give back the borrowed sim it will be fine now.
Full method in detail