Hi,
I would have thought that the mouse would work - unless of course you have replaced a PS2 with a serial. If you have done this you will need to edit the X config file to reflect the new device.
However, if the mouse was dead when the machine was started, then the mouse handling system may never have started.
Simplest thing to do would just be to restart X windows (I assume you are referring to a mouse in X rather than a console text mouse). You could do this by changing run levels but you would risk restarting services that run under the GUI run level 5 only that you may have altered from the default. Best way would be to close any applicatins you have running under X, then hit <CTRL><ALT><BACKSPACE> (not <DEL>). This will kill X windows but as you're in run level 5, X would start again - re-reaading the X config file to pick up the new, functioning mouse.