Why does my cell say I have no sim

There are a couple of possibilities: 1) you don't have a SIM. 2) Your SIM is non-functional. 3)You have a hardware problem with your phone.

The first thing is to try to remove the SIM and put it back. Sometimes they get dislodged just enough to not work. If that doesn't work, try another, known working SIM or get a new SIM from your carrier. If that doesn't work, contact Apple to have your phone serviced:

Genius Bar Reservation and Apple Support Options - Apple

Is this a new phone, or were you using this phone and all of a sudden this error appeared? If so, start here, If you see 'invalid SIM' or 'no SIM' on your iPhone or iPad - Apple Support