After restarting my phone, it's not recognizing my SIM card anymore

[AC Question]

Alright, so today when I went to testing, I had to turn my phone off, however, when I did and then I turned it back on, I saw the big circle with a line through it indicating that I had no service (I didn't have the ability to text, call, access 4g, etc)

I have taken the SIM card out, and put it back in.
I have restarted my phone
I even completely factory reset my phone, all to no avail.

However, when I restart my phone, I see a new "unlocked lock" symbol on my screen, and text that says "Custom" under it.
I have never seen the unlock sign nor the "Custom" text on my screen before, could this possibly a hardware problem, if so, how in the world did turning on and off my phone do this.

Along with all this, when I factory reset my phone, it seems to randomly restart itself every now and then.

Please help!

 

[Rukbat]

The OS is corrupted. Bring it to your carrier and carefully explain that ALL you did was turn the phone off, then turn it on. If they're nice, they'll reflash the ROM. (You'll lose anything you don't have backed up to another device.)

 

[Starm1X]

@rukbat Hey, this is OP, I just had to make an account to post a response, anyways, is there a way where I can myself flash a ROM onto it? And could installing a new ROM/OS void my upgrade plan with ATT? The contract is over so I'm wanting to upgrade it Tuesday (5/17), is it worth the hassle to try to install it myself and possibly void the upgrade plan, or should I just wait for 5 days?