Hi Customer (name blocked for privacy).
There's a fair chance that your thermostat has failed. We had the same experience with our old fridge. You can buy a replacement thermostat from a refrigerator-servicing company and fit it yourself, if you are mechanically-minded: they have generic thermostats that fit a variety of older fridges. Or you can get a fridge serviceperson to do the job.
Hope this helps.
Regards,
Sue.