Access will allow you to execute an INSERT
statement which adds a row with an unused autonumber value.
INSERT INTO MyTable (auto_num_field, text_field)
VALUES (27, 'foo');
However if Access objects due to a defined relationship, you need to first drop the relationship, add the data, do whatever else is needed to satisfy the relationship constraint, and finally re-create the relationship.
OTOH, if you've already changed the field's data type from autonumber to something else, and you don't have a backup copy of the table, this could be even more challenging. We'd need more information to figure out a solution.