Sorry for the less information I can offer since I am just able to access internet in Windows rather than in Linux now.

I am using Thinkpad T420 with Debian testing. My netcard works well before and it begins to work straightly these days.
It is suddenly down sometimes and I have to reboot the system to get it work. Sometimes the light of netcard is also down but sometimes keeps on(but I can't still access internet)
I read the /var/log/message and found that:
e1000e: NIC is up to 100Mps ...
...
e1000e: eth0 NIC is down

That's what I can remember. I googled it and found no solution except replacing the kernel.
Any suggestion?
BTW: the internet works well in Windows 7.

check ifconfig output,
you can try "service network restart" (as su) if it network down.