Why won't my Nokia Lumia 520 (windows Phone 8.0) read my sim card?

[Windows Central Question]

my nokia lumia 520 (windows Phone 8.0) will not read my sim card

my card is activated it just shows the sim card is missing or invalid

 

[jmshub]

Re: my nokia lumia 520 (windows Phone 8.0) will not read my sim card

Make sure you have the SIM card installed in your phone before you power it on, or else it may not read it properly.

 

[xandros9]

The card or card reader could be damaged (worse case scenario)

 

[MDK22]

... or the SIM card is bad, or the SIM card is upside down (I know it only goes in one way) or, could you have the SIM card in the microSD card slot ?

Is it an AT&T SIM card, because the Lumia 520 comes locked to AT&T (unless you unlock it, or bought a factory unlocked phone). It'll work w some MVNOs, but ...