done these:
1.Update your iPhone to the latest version of iOS (7.1.1).
2.Toggle Airplane mode On and Off. (doesn't work)
3. Try turning iPhone off and then on again. (works for a minute)
i recently replaced my sim to a nano one. And asked them to turn the gprs off cause it spiked my bill
Read more: What else do i need to do? Iphone 5 'invalid sim' problem?