There are different rules in various jurisdictions. Here's what we use in California

1544. Calling off Race.
(a) If the stewards determine a race cannot start before midnight or cannot be conducted in accordance with the Board’s rules and regulations, they shall cancel and call off such race.
(b) The stewards may declare a race no contest if mechanical failure or interference during the running of the race affects the majority of horses in such race.
(c) Any wagers on races called off, canceled, or declared no contest shall be refunded, and no purse, prize or stakes shall be awarded.
(d) If a race is called off, canceled, or declared no contest, any submitted claims shall be void.
(e) A race shall be canceled if no horse covers the course.