Why would my tablet not accept my sim card anymore

my Tablet was working fine with no problems at all, about a month ago it just did not want to go on to the internet anymore. i thought is was my data so i loaded 3 gig on my sim at cell c, when i put the card into my tab there was still no responce fromthe internet, i then went into my settings and selected the network and it said to "insert your sim card" whitch was in already, i took it out and inserted it again and still the message come up. i tryed to put in another sim and still got the same message, please help