A single blocked toilet can be due to excessive amounts of paper, wipes or a toilet freshener that gets stuck in the s bend. This can be normally removed with a plumbers plunger or special tool called a sani snake. If the toilet still cannot be unblocked then it is probably roots that have grown into the outlet drain.
The cause of root blockages is normally cracks in the drain, previous maintenance or bad installation.
A cleaning access is generally located under ground and the toilets outlet drain is scoured clear of roots with a drain machine from the access, the drain is inspected with a drain camera and a root control system option can be applied.