Undermounted sinks are a nightmare.
Usually the sink is installed over chipboard or MDF, with the lip of the sinkholding the sink in place.
The problems begin with when the silicon seal from the benchtop to the sink is not maintained and water gets into the chipboard or MDF.
The sink just drops away.
To fix, we have in the past supported the sink with a timber or steel frame to hold the sink in position and re silicon.
Not much else you can do without removing the benchtop.