1) Make sure you're using a USB cable that supports data - some, like the nes that come with Bluetooth headsets, only supply power. There are no data wires.
2) If you're using the original cable, try another cable from a Samsung phone. Your cab;e might have gone bad.
3) Your microUSB port could be broken.
4) Make sure that plugging another Samsung phone into that computer works. The driver may have become corrupted.