The reason that it is not connecting can be anything from wrong driver installation to the dead Atmega 16u2 chip. But I must exclude the former option as you said you'd tried on different OS and different computers. So double check the driver installation. And if does not work try to find out whether Atmega16u2 is fried or not. One solution might be to reflash the original firmware through an external ICSP programmer. If don't have any programmer but another arduino uno you might try out the gamon's tutorial on flashing firmware on the little USB to serial interface chip.
Anyway if your arduino is new and official you can get the right support online on arduino.cc. but if it is a clone you may try to contact the reseller whom you bought from.