Ethernet cables can last years and years but the connectors can get dodgy over time. To be honest, the little plastic clip bit normally breaks and the cable gets thrown out before it's so old that it fails anyway. The plastic coating around the cable should be inserted completely inside the connector. When you wiggle the connector, the cable should move as well. If you can turn the connector 90 degrees from the cable, then it probably has worked itself loose over time and one of the very thin 6 or 8 strands inside the cable may have broken off of its terminal inside the plastic connector.

If nothing else has changed on your network, then it would certainly be worth swapping out the cable just in case - it's cheap and easy. Has the networking configuration on the PC been altered or has any new hardware been added inside or outside of the PC recently? If so, then it would be worth checking that out as, like I said, it's not very common for a cable to stop working.