if your running Vista, its putting your ethernet/whatever to sleep if you can not reconnect without restarting you computer.
Its a easy to fix if thats it,
1)Right click computer(my computer)>
2)Properties>
3)Click device manager>
4)Expand the Network adapters(hit the +)>
5)Right click then properties on one of them>
Power management>
uncheck everything
6)Back to 5 until you do it for everything under that.
Only problem, arg one bug i hate, sometimes you need to have it checked! But try to do it with the adapters power management settings all unchecked first! Try the list above first tho instead of doing the reverse ;p
Also set your computers power saving(sleep) to Never. Having it on a time, makes above wrong/right/whatever it acts weird but i have no problem with power saving(sleep) + having the power saving check boxes off in the device manager.
If your not running vista, ignore this post