Need more info.
What are you connecting the PC to - router ? If so, are you able to access the router settings menu ?
Is your pc standalone, or part of a home network? If the latter, can you see the other machine(s) ?
Are you currently using wired or wireless ? If wired, have you tried a different cable ? In my experience, network cards are pretty reliable but cables can fail unexpecredly.
Are you getting any carrier or signal lights on the network card or at the other end ?
Write back with more.