1st Gen iPhone - Screen says "NO SIM" but i do have it installed

I was trying to charge my 1st Gen iPhone (has the latest update)(NOT JAILBROKEN-NEVER) and when i plugged it in the "AT&T" symbol at top-left corner changed to "NO SIM." I immediately tried making a call and it wouldn't work. I tried restoring the phone but when it rebooted it had the message. The phone would let me log in & out w/my password and i was able to use the internet but as soon as i hooked it back into the computer to see if it needed an update which it did...it gave me this message:

"No SIM card installed - Insert a valid SIM with no PIN lock to activate iPhone."

Now it doesn't even let me log into the phone...the slider says "Slide for Emergency." When i do slide it it just gives me the the key pad. When i hit the info button it gives me this message:
IMEI: 01 136400 159404
ICCID: Unknown

I've tried hooking it to iTunes and now iTunes can't recognize the SIM Card it keeps telling me to make sure there's one inserted.

I've popped out the SIM card and pushed it back in but still get the same screen. I've rebooted phone manually and same thing. At this point I don't know what to do. Please help w/any possible advice. Thx

It could be either the simcard or it could be a hardware issue with the phone. Inside the phone when you insert the sim there is a tiny plastic sensor that tells the iPhone there is a simcard in the phone, If that sensor breaks it will read "NO SIM" even if a simcard is in the phone. I would try a new sim first.

I had this problem recently as well. I actually got my old sim card from my blackberry and put it into my new iphone. Then randomly it would say NO SIM CARD even though it was in there. I would have to turn off my iphone pull it out and then put it back in and keep doing that until it worked. So then I just went into the ATT store, they looked at my SIM card and then they said it could be the SIM card. So they were nice enough to give me a new SIM card for free and now I haven't had any problems since.

Had the same issue, i was told that some sim cards are just faulty or that they can become faulty

If you go into an AT&T store and tell them that you think you have a faulty sim card and you have reset the phone etc. They will give you a new sim card, takes about 3 minutes, worked for me.