My iphone doesn't have a sim card?

I have an iphone 5 (with at&t) right now (been using it since January) and I tried taking the sim card out only to find out it doesn't have one

I put the sim card slot back in but my phone is showing a "No SIM Card Installed" message and I have no idea what to do since apparently I didn't have one in the first place

Any help?

I would ask at&t. It's so small maybe it fell as you tried taking it out

Look for it, it fell out.

It must have had a SIM card. It would not work otherwise.