If everything was working, then you edited a theme file and got a 500 error, it is most likely you messed up some code, revert whatever you did.

If you switch to the default theme and disable all your plugins and the 500 error still appears, contact your host and/or check your error log, you might have to turn on more error reporting depending on what level it is set at.

To have multiple menus you can use register_nav_menus Reference:http://codex.wordpress.org/Function_Reference/register_nav_menus

To add them to your template you use wp_nav_menu Reference:http://codex.wordpress.org/Function_Reference/wp_nav_menu

Contact your host and/or check your error log, you might have to turn on more error reporting depending on what level it is set at, it could be any number of things including memory allocation.

To have multiple menus you can use register_nav_menus Reference:http://codex.wordpress.org/Function_Reference/register_nav_menus

To add them to your template you use wp_nav_menu Reference:http://codex.wordpress.org/Function_Reference/wp_nav_menu