One of the 3 multisites has decided not to work now.

Any 500 error means “internal server error” and can be any one of 1,000’s of different things.

The best way to debug is to look at the sites error logs. They might be able to show where an error is occuring. You can also change this line in your wp-config.php file from:

define('WP_DEBUG', false);

to

define('WP_DEBUG', true);

and that will show errors instead of hiding them. Just remember to chagne it back when you’re done!