The dreaded possibility is bent SIM card pins in the inside of the SIM card slot itself. I trust you have already tried out different SIM cards just to be sure the one you have always used in the tablet did not just go bad (as they sadly do fail out of the blue). You can have it disassembled and the SIM slot inspected to see if the six pins are still all present or there is a need to re-solder.
In the extreme, there may be a bad resistor requiring replacement as well. If the tab is insured or still withing the 1 year warranty period, I would suggest you contact your retailer as the fix to the issue should be essentially free of charge.
Do let us know should you have any further need for assistance on this very selfsame issue.