|1. Bad cable|
2. Wrong cable type. Make sure you have a straight ethernet cable and not a crossover (unless auto cross capable switch is used)
3. Bad network port
4. Switch port is shutdown or disabled
5. Network port on computer disabled
6. Network port on switch and computer are not configured correctly. Try setting both to auto speed and auto duplex while troubleshooting.
7. Switch port configuration is not in "access mode" meaning it could be configured to be a trunk port (as an example)
8. Access control list in place such as MAC filtering and your PC is not in the access list
I could keep going but the point is, all you have said is that you have a computer that doesn't work. A bit more information might get you an answer.