Normally the loss of a hydraulic system will involve some degradation in capability depending on which system is lost, demands are usually spread out so that the loss of a particular system won't result in a complete loss of function. For example, the engine thrust reversers are generally supplied by different hydraulic systems on each side so in the event of a failure one reverser will still be available.
Critical parts, such as the rudder will have dual or even triple sources of supply as they have to work.
The decision to return or divert will be influenced by the destination airport, for example a short runway wouldn't be compatible with reduced stopping performance and not having any maintenance available could result in a prolonged grounding of the aircraft.