I can only relate the experiences with fridge/freezer combos here in the U.S., so if this isn't "spot on"... apologies. However, I expect one thing to be similar, that being their is only one cooling source. The actual workings provide cold air to the freezer and then a fan along wiht a thermostatically controlled door device allows some of that cold air to be blown into the fridge section. The thermostat control in the fridge section serves only to control the amount of cold air coming through. So, I would expect the door device or circulating fan to have failed or possibly the part of the thermostat that controls all of that...
You may be able to manually move the small door device to see if you can, temporarily, get cool air into the fridge but sounds like a technician is required...
Best of luck!