in my case, poor/bad cables, especially the ones you buy, rather than ones you make, rank very low on things that actually happen. before I check the actual cable I do other things:
1. see if nic led is on, both on router and computer
2. ping the gateway
3. see if there was traffic on the nic
4. re-seat the cable, especially if plastic clip is missing - if that is the case I change the cable.
5. if distance is short, replace cable
had an interesting experience with a bad "connection". One summer the well pump, which is drilled inside the old stone well, 80 ft below ground, stopped working. The plumber comes, he climbs half way down the stone well in order to reach the well-cap, takes the entire pump+hose+wire (80ft) out, tests the pump, concludes that the pump is dead, replaces the pump, the wire (80ft), and makes a new water-tight connection. A year later the pump stops working again and I call the same plumber. My instinct was that the water-tight connection, 80 ft below ground, leaked, and made a short. The plumber comes, and rather than pulling out 80ft of pump to test the water-tight connection, the first thing he does is disconnect the pump from the mechanical fuse. And guess what... the mechanical fuse was bad. The fuse would trip even when there was nothing connected to it.