Have you checked the cable(s) you are using.

1. Make sure it is not a cross over cable
2. There may be a break in the cabe
3. The distance is just too far

to verify this yuo can try changing your NIC speed to 10MPs half duplex, to do this, right click my network places, right click on your nic, click Configure up top, then advanced, then media type and Choose 10mb half and see if it connets

if it does you are too far or are using a lousy cable

good luck

since 3 days i have the same problem with my inspiron 8200,
it worked also fine before - with the same cable????!

flo

Thanks for the advice lcohen.  I tried three different cables, and made sure they weren't crossover cables.  One thing I forgot to mention is that the activity light/s weren't on at all at either the connection on the Switch or the NIC (at the RJ45 jack).  Anyway I called tech support and they ended up having to send a tech to our office to replace the motherboard (since the NIC is built-in) as well as the RJ45 jack, thankfully still under warranty.  It's a good idea to have a spare external network adapter for when something like this happens, I happened to have a Linksys USB network adapter that worked well, while I waited for the tech.