Why does my digital paraffin bath not heat up anymore?

Always a good idea to include the model number of a unit in your queries. :-)

Short answer (but not necessarily **a short** lol):

Heating element has died

Malfunctioning circuit board component(s)

Malfunctioning switches

Connections loose or have to be reseated for proper connectivity

Power input malfunction (example broken AC cord which plugs into household receptacle.