HELP! broke something off my mobo!

It's a capacitor, used for smoothing supply voltages. If they are through-mount (the pins go right through the motherboard), then any electronics guy can replace it or solder the old one back.

Your mobo could keep on working without any issues too. Perhaps you are not using the component whose power supply the cap was taking care of or maybe the power is smooth enough and doesn't need the filter.