The alarm light is normally illuminated in the following circumstances
I would start by checking if the drinks chill function is on ts a bell icon, just above the temperature on the LCD display.
If the drinks chill function isnt on, the most likely cause is the freezer compartment is too warm. Check the door seals are undamaged and making good contact all around the appliance. If this is the case I would be included to switch the fridge off and back on to clear the memory, and see if the warning light comes back on.
This sounds like as though there is a lack of coolant reaching the fridge and freezer, this can be caused when there is a build up of frost in the coolant duct, due to the defrost sensor or element failing.
Its worth performing a full defrost over 24 hours, before booking an engineer in to inspect the fridge freezer, as its difficult to identify a fault if there is a lot of frost present when they remove the back panel.
If you would like to proceed and get an engineer booked in anyway, to check the coolant and defrost system, please advise of your post code.