need help on win2000 server internet connection

hi guys, im new in this forum seeking answer on how to solve a problem on our small office network, we are using win2000 server, all is well until one day, the lancard for the dsl connection is not accepting packets but the lancard for the internal network is fine, when i interchange their role, still the same, the lancard for the dsl is not working while the lancard for the internal network is fine, but when i disable the lancard for the internal network, then i can browse the internet from the server, do you know the solution? i have tried everything but in vain. thanks a lot for any advise!