Parent never knew about Hearing. Never received the Summons. The Hearing went ahead. How can this be deemed as a Legal Hearing?

Maternal grandparents got an Interim Residency Order through Court in 2010,  Mum did not oppose it.  Mum and child have been in constant contact, Mum has always agreed the best thing for the child is to remain in Maternal Grandparents care as long as the Contact between Mum and child stays the same, which has been very positive.  Mum thought everyone was in agreement.  Childs School Head Teacher mentioned that in 01/2015 there was a new Residency Order through Court as School had no record of Interim Residency Order.  Mum was never Summoned to Court,  there was a Summons though Court had Mum's last address.  Maternal Grandparents could of told them Mum's new address...How can any of this be Legal when Mum was not aware of Court proceedings, and never signed or saw Summons so obviously Mum never went to Court as had not signed or saw any Summons and also never knew there was any Court proceedings???