Try removing the Driver through device manager and heading over to the manufacturers webpage and re downloading
Fail that, Have you tried a different cable?
Best bet might be just to grab either a USB - RJ45 Adaptor
Or even a PCI Internal Ethernet Port, The onboard might of carked it, Ive had VERY Similar problems and it was just a pest, a different port solved all the problems, Bad co-incodence but it might of died in transit