Null IMEI and no mobile network

[Cattich]

I bought an unlocked t mobile samsung s3 4g lte and it was working... then a few months later it says no mobile network available. I reboot my device and nothing. I checked solutions online and tried the *#06# and my IMEI is null/null. I went to mobile networks and it says to insert USIM card to access networks when there is already a sim card there... I tried the holding the power & home button thing and still nothing.... HELP ME!!!!

 

[Almeuit]

Did you buy it used? It sounds like the IMEI was reported / blacklisted.