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Power down. Grab a sim removal tool, or a stout paperclip. Remove sim tray and reinsert. Power on and hope for the best. Next stop would be the carrier for a new sim, or apple store.
 
OMG, my iPhone 5 was working PERFECT until now!

HELP!!

I had the same issue for the past 3 days. This morning, I went down to the local AT&T store for a replacement SIM card. They installed a new SIM card in some 5 minutes. Hopefully, this will resolve the "no SIM card installed" issue.

There appears to be quite a few people, whom are experiencing the same problem, over at the Apple Community site, as well.
 
I've had sims go bad on me too. I don't understand why people panic unnecessarily. :rolleyes:
Power down. Grab a sim removal tool, or a stout paperclip. Remove sim tray and reinsert. Power on and hope for the best. Next stop would be the carrier for a new sim, or apple store.
 
Go get a new SIM card from your carrier, no big deal. As long as your screen isn't cracked or you dropped it in water, there isn't much to worry about.
 
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