If you set it to something your router/switch doesn't support, it won't work. Also, other computers on your network should be setup with the same settings to ensure they work together at the same speeds. Sometimes with 'forced' mismatched settings it may not want to connect. I have ran into that problem before.
If you have a gigabit switch, setup your network adapters to auto-configure (assuming they're gigabit as well), they will automatically recognize the proper speed for the switch you're running and it will go smoothly. The slower devices (10/100) will auto-configure to their respective speeds as well.
So.. I'd set it back to auto and give your computers a restart to see if you fix your connection problem.