She missed visitation because she is missing. It was reported that she never missed it prior.
It is not uncommon for two unmarried parents to have a verbal or informal custody agreement. Unless of course they were legally married, there was no legal requirement to have one. It is possible that what started out as an amicable agreement, turned contentious. For whatever reason, the father may have started denying her access. This would mean that she would need a formal order to ensure her co-parenting rights are being adhered to and specifics are set out to prevent further violations of parenting time.